Yes get specialist advice. Effectively trees (once mature) take water from the soil at a pretty constant rate. Removing a tree or adding a tree upsets the equilibrium and this can have a detrimental affect on the footings and foundations. Some trees are much worse than others (willow, eucalyptus for instance) and their root systems can cause havoc with old clay drains in particular.
The BRE have papers on trees and their correct safe distance from houses in relation to their height etc. Well worth a look if you can find it.
That's most useful advice, thanks!
It's a massive fir tree it seems, and takes up a fair amount of the back garden. There are quite a few other medium-sized trees in the neighbouring gardens (the garden's aren't wide) and also a couple of others at the end of our. I'd hope that these helped with the soil balance, but I'll definitely get a specialist in to take a look before we do anything. We're about 50m from the Thames, so it's bound to be a special case.