Fern Britain: A 19th-century fernery has been successfully brought back to life on a Scottish island

Wemyss Bay railway station, on the west coast of Scotland. That's where I am. But it feels as though I've been swept on to the stage of a Ruritanian operetta. In one corner, members of a silver band, wound round with instruments of monumental proportions, are belting out the kind of tunes that I seem to have known for ever. The station is a circular building, Victorian, mostly glass, with curlicues and fancy ironwork throwing extraordinary shadows on the immaculately swept floor. Big tubs of flowers line the side of the glass-covered promenade that leads to the ferry, waiting just outside. Train and ferry connect. Even that has now come to seem a marvel, enhancing the feeling that here, you are not in the real world.

It's a half hour ferry ride from Wemyss over to the island of Bute where, on the front at Rothesay, is the most spectacular men's lavatory in Britain. Or so they say. The man handing out fresh towels in the washroom (yes – proper towels) said he did tours when the place was not busy. I hovered for a while, hoping, but the opportunity did not arise. The outside, though, gives a hint of what that inner sanctum must be like. Very ceramic. Very beautifully detailed.

Instead, I walked along the front where curiously clipped cherry trees mark out a particularly brilliant piece of public gardening. Geometric beds in diamonds and crescents, with the earth humped up inside in the old way. Edges as straight and as clipped as Hitler's moustache. Turf immaculately smooth. And the beds themselves in full spring fig: pink tulips edged with forget-me-not and underplanted with brown wallflowers and cream polyanthus; purple tulips edged with red polyanthus and underplanted with primrose wallflowers and forget-me-nots; red tulips underplanted with scarlet polyanthus and golden wallflowers. Traditional elements yes, but the plants so beautifully grown, the effect so heartlifting.

This showpiece sits between the palatial lavatory and a lifesize statue of Alexander Bannatyne Stewart, "erected in affectionate remembrance of his public services and benefactions to the county of Bute". Stewart, shown gazing proprietorially along the southern shore, died in 1880 and he's the reason I've come to the island. This was where he had his getaway place, sailing down from Glasgow in his steam yacht to spend his summers at Ascog Hall, built with the proceeds of his shipping business and his stores.

Ascog is only a few miles down the coast from Rothesay and the road is a delight, lined with preposterously splendid villas with deep eaves and extravagant ironwork. Stewart's own house is just what you'd expect – plenty of turrets. The surprise is in the garden where, around 1870, an extraordinary fernery was made by excavating a huge hole, lining it with rockwork, then glassing over the top. The roof rises up from ground level, curving elegantly like the top of a canvas tent, the ridge decorated with more of the ironwork typical of glasshouses of the Victorian age. This one was made by James Boyd & Son of Paisley.

Red sandstone rock was hauled up from the beach just the other side of the garden boundary to line the walls of the underground cavern, with planting pockets for ferns in between. At the far end, a waterfall tips into an undulating central pool skirted by a path cobbled with small white stones also brought from the beach. When I was there earlier this month, the great fronds of osmunda and pteris, blechnum and cyathea were just beginning to uncurl, like snakes rising from their nests. It's the ferns' great moment. They are never less than wonderful, but in this particular instant, the rebirth was a mesmerising, magnificent display of Zen backflips.

Susannah Alcorn and her husband Graham are the lucky owners of Ascog Hall, but, as she explained, it was her mother and father, Katherine and Wallace Fyfe, who did all the hard work in restoring it. Kath, brought up in a Glasgow tenement, came to live on Bute with her husband and, quite suddenly, got the gardening bug. Soon she outgrew the garden she first made there and started eyeing up Ascog Hall, then in a ruinous state. The fernery roof had fallen in and the ferns were so covered up, there was no way of seeing they had ever existed. Wallace was an electrician, "practical, positive, optimistic", says Graham; even so, this was a monumental place to take on. But the Fyfes bought Ascog Hall in 1986 and moved in six years later. Which is when they discovered the fernery filled with 16 tons of accumulated rubbish, broken glass and mud.

Taking a deep breath, they started work again, shovelling out the debris with plastic snow shovels, so as not to harm the pebble path, and organising a completely new roof with hundreds of panes of glass, none of which was the same size as its neighbour. Wallace used his concrete mixer to make special compost for all the planting pockets and by 1997 the whole fern house was restored and replanted.

Although the original rockwork was in surprisingly good shape, only one of the original plants had survived, a massive South African fern, Todea barbara, with a trunk so huge it looks like a sheet of rock. A correspondent of the Gardeners' Chronicle described it in 1879 as a "most wonderful plant" (which it is) and reckoned the trunk measured at least 5ft across, 15ft round. That same report listed all the other plants in the fernery, which gave the Fyfes the shopping list they needed for the grand re-stocking.

And meanwhile, of course, Kath was gardening furiously in the three acres that surround Ascog Hall. Originally laid out by the Victorian decorator and garden designer, Edward La Trobe Bateman, who was working up the road for the Marquis of Bute, the wide gravelled paths and rockeries had, like the fernery, disappeared under mounds of bramble and sycamore. But she reclaimed those acres and packed them with her favourite plants – fritillaries, roses, blue meconopsis. She died last year, a year after Wallace, leaving their only daughter, Susannah, with this extraordinary memorial. She's overwhelmed of course, but not so overwhelmed as she might have been if she had not had the good sense to marry Graham, who has gardened at Mount Stuart, the Marquis of Bute's place, for the past 20 years. When the Alcorns' three young sons are off to school, I bet it won't be long before Susannah gets the bug too.

Ascog Hall, Ascog, Isle of Bute, PA20 9EU is open Wednesdays-Sundays (10am-5pm); admission £4