Quick update:
Found the source of my suspension knock - the driver's side rear coilover had been leaking and the damper was fapping about doing nothing! Thought about rebuilding but as they are the JDM Teins the car was imported with thought it would just be easier to replace, so went for some nice new BC Racing BR series. To say the car is transformed is an understatement! Not only has the knock gone but the car now glides over bumps, all the harshness I would usually associate with coilover suspension has now gone - I think the decision to choose rubber top mounts rather than pillow-ball style ones like on the Teins helped a lot!
In other news, the Supra has gained a new stablemate
Factory Midnight Purple 180sx Type X - allegedly the only one in the UK in this colour!