Hi Gillybean. We're having a very similar problem. 1930's house with a three year old two storey extension. Cracks are appearing all over the place, particularly along the wall/ceiling boundary and a few diagonal cracks both upstairs and downstairs in the original house. How did you get on with yours? Any advice re- seeking a structural engineers report or insurance claim ? Regards, Mark.
To be honest I have just taken photos and monitored them, nothing major has happenned it's just annoying as soon as you fill and decorate they gradually open up again even with 'flexible' filler. I haven't gone down the road of structural engineers etc it sounds a whole can of worms. There was an interesting programme with Sarah Beeney about diagonal cracks and that the opposite of the 90 degrees of the angle points to the problem or something like that! I am sure the highway tree is our main problem and the fact we are on clay doesn't help. Good luck!