Fantasy figures: Isabel and Julian Bannerman reveal their enchanted garden

The respected landscape designers show Anna Pavord around their extensive Bristol back garden

I used to dream that one day The Independent might do a garden at the Chelsea Flower Show ("Dream on," I can hear the editor mutter). I had it all planned out in my mind. When the command came down from on high, I would cry "Bring in the Bannermans", knowing that a Gold Medal would inevitably be in the bag. Garden designers Isabel and Julian Bannerman have a big fan club and I've been a member as long as I can remember. But since bringing them to Chelsea this year is not an option, I did the other thing and went to their own garden instead, open regularly this year for the first time.

"Really, really daunting," says Isabel of Hanham Court, the ruinous collection of buildings they took on in 1993 on the eastern edge of Bristol. The place was owned by a brother and sister who didn't talk to each other and insisted on separate viewings for their separate bits of the house. Chinchillas and Great Danes roamed the grounds in a thicket of brambles, hideous conifers and snowberry; the roofs of the barns were more gone than not. But the buildings dated back to medieval times and once beyond the conifers, the Bannermans could see that the way the land lay, rising on one side of the buildings, falling on the other, presented great "capabilities" as Lancelot Brown, the 18th-century garden designer used to say.

The transformation at Hanham Court is spectacular, even by the Bannermans' standards. If you've seen the work they have done at Houghton Hall in Norfolk or the recently completed "Collector Earl's Garden" at Arundel Castle in Sussex, you'll have an idea of their style: rich planting, romance, surprises. And a very clever way with green oak, used in quantity to make the kind of follies, obelisks and balustrades which you usually see built more ponderously in stone.

The way in to the garden at Hanham Court leads through a small courtyard with iron railings and an archway which links the house on one side with the medieval church on the other. The view ahead is over a long, thin rectangular lawn, a Tudor bowling green, which drops away at the end into wild meadow. Immediately in front, vast mounds of Euphorbia characias with the biggest lime-green heads I've ever seen, grow straight out of the gravel, with fans of Iris pallida and feathery didiscus in between.

If you force yourself to turn away from this (it took me a long time – there is so much to take in at this point) you have another breathtaking view: a long, straight path leading to steps and beyond the gate, a walnut tree stretching out its huge limbs to touch the rising ground around it. Underneath, vast quantities of snakeshead fritillaries and old-fashioned narcissus push through grass and cow parsley giving the kind of "natural" effect that is in fact the hardest of all things to achieve in a garden.

Rough pasture, mostly planted as orchard, rises quite steeply ahead, but it's worth getting to the top because then the more formal part of the garden is laid out like a map below you. You see then, too, that the long bowling green and the borders either side of it are all contained in a huge and ancient retaining wall, a bastion running out like a long apron in front of the house to make level ground where originally there must have been quite a steep slope.

The walnut expedition over, you can take more time over the things you missed on the way out: superb Paeonia rockii either side of the 1730s gate, with buds as fat as artichokes (the whole garden is lush with peonies), mounds of expertly staked delphiniums in the border backed by a tall hornbeam hedge. If you turn left off the path through a green oak pergola, you emerge in the box garden with a centrepiece of box balls and enormous pots of tulips. The pots are more than usually tall, which brings the flowers up just where you want them to be. But the depth of soil under their roots suits them too. Another idea to copy, I thought.

Against the back wall is a monster stone trough with square box-edged beds either side. Box-edged beds are a familiar sight, but the Bannerman brainwave is to top the box hedges with broad planks of green oak, comfortably bevelled off at the edges. So the hedges become seats as well and the box grows happily underneath making little green-leaved walls under the broad wooden tops. The beds were filled with 'Jan Reus' tulips – one of my favourites – deep, rich, maroon-red flowers which last an unusually long time. The Bannermans have experimented with various different combinations to take the beds through summer. This year they'll be using scented-leaved pelargoniums mixed with mignonette.

When I was there, it was too early for flowers on the roses, but just the right time to study how they had all been trained. There are masses of roses, stretched out over stone walls, piled up in the borders, and they are superbly trained and tied in. The old-fashioned roses that the Bannermans like can be rangy, difficult things to use in borders, but they've had excellent supports made: strong hoops of iron, two or three circles piled up above each other on long legs which can be firmly driven into the ground. Then the arms of the roses are twined and tied in round the supports to make big drums of growth. This way, they keep themselves out of the delphiniums' hair.

Some climbing roses can be leggy things, giving you at eye level nothing more than bare stems. The Bannermans use tall thin shrubs such as Daphne bholua 'Jacqueline Postill' to disguise these stems and give early spring interest as well as the summer splurge of rose blooms.

The formal garden that runs down the side of the long lawn is an intricate, interlocking series of spaces that includes a swimming pool and a grotto, but I was mostly looking at the planting: double-flowered 'Buckeye Belle' peonies underplanted with sweet williams and lilies. The foliage of peonies is so good at this time of year – lush, shiny, bronzed.

On the other side of the lawn, the land drops away steeply to a completely different kind of garden, a woodland slope rich with ferns and martagon lilies, cyclamen and Solomon's seal. Two enormous copper beeches tower over bushes of sweet smelling Viburnum carlesii with a tantalising glimpse beyond of a whole forest of tree ferns and the impossibly huge goblet flowers of magnolias – massed magnolias. But the Bannermans think big and the biggest surprise is hidden down here at the bottom of the slope. I'm not going to write about it. You have to go and see for yourself. This is a garden that you would be mad to miss. It is open Friday to Monday (11am-4.30pm) until the end of August. Admission £5. For more information about Hanham Court, Hanham Abbots, Bristol BS15 3NT, go to the website at hanhamcourt.co.uk.