the links worked for me also - hear a faint clunk - I dont think it is consistent enough to be in the diff itself or a bearing as they tend to be more cyclic or grinding - Has the rear suspension scooby knock been ruled out??
As you have checked all that I would be bolting a go pro underneath and taking it for a drive to see if anything moved prior to going into a garage...?
great to hear it pulled you round - as said, testament to your reactions and a well maintained motor - and did you get any of your knocking from the rear ;)
That is the car I should have gone for instead of the cavalier as the Mk1 Astra was a solid car - had a few faults I never got to the bottom of but solid none the less