I have fixed it, temporarily. Many thanks to your help.
I explored many of the points identified and whilst poking around I had a good tug on a large metal pipe under the passenger seat that runs near to the chassis, into a rubber hose that comes up into the Turbo (I think that is what it is) so presume it to be the air inlet. The resulting noise was familiar! Down by the chassis, a bit awkward to see, there is a bracket welded to the pipe that screws to the chassis through a rubber bush which was worn. I have done a temporary fix using some rubber grommets whilst I obtain a proper replacement (not sure where from yet!). All the work was done from underneath. Thought I would add some pics, partly because I feel the need to try putting pics up here .... and they might even be helpful.
The offending worn mounting viewed from underneath the passenger footwell with the under engine cowelling dropped down.
The mounting removed for inspection and showing it worn through
The rubber bush I now need to replace
Trying out an ordinary rubber grommet for size
Bodge constructed from various grommets and washers to get as close as possible to the spacing of the original
Fix in place with cowelling back
I gave the exhaust clamp by the gearbox a little tighten as well.
All that was left was a test drive. All symptoms gone including the clatter when turning the engine off.
thank you to everyone for your ideas and thoughts.
... now to check those air con pipes ....
