mysterious clonking

somewhere in the drivetrain when I go over a bump. not every time and I don't think its the diff, its not the driveshaft or gearbox.

im thinking its to do with the suspension. maybe the spring bushes? how can I tell if its the spring bushes?

You can visually inspect them. You will see if they are worn. Also check the rubbers that hold the shocks on, top and bottom. The spring bushes have 2 metal sleaves with rubber inbetween, that rubber will have shrunk and look all old, or possibly seperated from the metal sleaves if they are worn. Use your own judgement tho, will be quite obvious. You will get a general idea of there condition without removing anything.

If you get a long bar between the spring and the chassis at each end of the spring and give it a bit of leverage you should be able to see any wear
also check to see if the u-bolts aren't loose,mine had a bad knock and it turned out to be loose u-bolts