Try Andrew Dust Structural Engineers:
http://www.andrewdust.co.uk/
I found a good Telegraph article on this.
http://www.independent.co.uk/money/crac ... 66696.html
Quote from the article:
"The BRE has drawn up a classification of subsidence damage with recommendations for action. These are set out in a useful layman's guide, Has Your House Got Cracks? by TJ Freeman, GS Littlejohn and RMC Driscoll (published with The Institution of Civil Engineers, price pounds 9.95.) Category 0 means hairline cracks causing cosmetic damage only. If you are unfortunate enough to have cracks wider than 25mm, or the walls lean badly, the windows break and roof timbers lose their bearing, your house is in category 5 and may even fall down.
Category 2 signals the time to take action. This is when you have several cracks at least 3mm wide, possibly on the inside only, that do not close up in winter. The solution for this fairly minor damage could be as simple as pruning or removing thirsty trees. Trees are at least partly responsible for about 80 per cent of all clay soil subsidence claims. No tree should be planted nearer than 5m to a house, and willows should be at least 40m away. But pause before you reach for the axe. Removing trees may cause more problems than it solves."