It was one of those eureka moments you get when you turn the telly off before going to bed, and sit quietly finishing the last glass of wine... The whole plot, apart from the area around the old greenhouse, has got good, deep top-soil, up to three foot deep in places. The area around the old greenhouse on the other hand has fairly poor topsoil going down 1 foot or less.
I don't know what the relationship between top- and sub-soil is in a plot cultivated long term, but expect that depth of top-soil will increase with the action of worms and micro-organisms as organic matter is introduced - perhaps turning the top few millimetres of sub-soil into top-soil every year. The old greenhouse hasn't really been properly gardened - dug over and fed, and planted out with plants with long roots, - since before the days of the greenhouse, which goes back several decades, maybe almost a century.
Thus, if you looked at the garden in profile, you'd see a hump of sub-soil where the greenhouse foundations are. Just to the south of this (the middle ground of the photo) is a depression, about 4 yards by three, and 1.5ft deep, approximately. I had a bit of a dig around in it yesterday, and despite the fact that it's two spits lower than the surrounding ground, it's still topsoil for at least another spit down... I had earmarked this area for the pond, but it means I'm going to have to dig down a good way, use a lot of clay subsoil from elsewhere, and the resulting pond is going to be way below the level of the rest of the plot.
The answer's obvious really. Excavate the top-soil from the old greenhouse foundations, (and the foundations themselves), use the spoil to fill in the depression on the photo, and then excavate the pond from the hump of clay that's been left under the foundations, thereby turning an oddity and an eyesore into a thing of beauty - an almost completely natural pond, with clay banks, marginal plants, frogs, dragonflies, god knows what else.