Jacked up the car today and 'attempted' to tightened the subframe bolts.
Good news is, the diffuser didn't need removing.
The weird thing was, all the bolts seemed solid, and tight enough. Had a good look around and couldn't seem anything amiss regards breaks, shocks or subframe.
Tightened every bolt I could find and finally gave up. My miss's needing dropping off in town, so I took the Supra.
Low and behold, the knocking has gone. But i feel this only temporary joy, as i didn't drive fast or take it on bumpy roads. A more rigorous test is needed, but fingers crossed.