So... I went to pull out the lower engine mounts, but when I went to undo the top dogbone, one of the bolts wasn't done up real tight. I thought, that wouldn't have helped. Anyway, I was already half way through. Up on jack stands, wheels were off, underbody tray was off, so I decided to pull out the lower right mount anyway to inspect it. The clunk was coming from the RHS and you could bring it on with throttle on and off. I hadn't bought mounts yet, so I thought I'd better make sure it was that.
The mount was fine. Not split. Maybe not quite as stiff as new, but couldn't see anything wrong with it. It has rubber that the top, not sure if the bottom is fluid filled. Couldn't tell by shaking it. Wasn't leaking at all.
Didn't pull out the centre or left, but undid the bolts to get the right one out.
Put it all back together (with everything tight - didn't bother getting the torque wrench out), went for a test drive and all is good. No clunk. Wasted a couple of hours that would have been spent on watching TV series anyway.