Hey all,
Thanks for the numerous replies - I'm now home and have some further info.
I've been under the hood trying to rock the engine as Gavin suggested - nothing shifting at all there. The exhaust looks central in the exit hole so that looks good. I've looked at the mounting brackets on the sides of the engine and given both a good pull in all directions - nothing shifting there either. The only thing I haven't, yet, done is to try and jack the sump up - the reason being it's so low I can't actually fit the jack under it!
While I was out there, though, I noticed the front suspension is somewhat slanted - possibly explaining the v low sump, if not the loud clanking noise.
Images are:
brackets:
here
here
here
here
exhaust:
here
and the suspension:
here
So - unless there's any good reason not to I'll try adjusting the dampers and see if I can get the jack underneath the sump (with some wood) once that's done. Otherwise it's off to buy axle stands - possibly these ones