Cheers bud - you've saved me a shedload of time :-)
Turned out to be the 33uF 50V cap above R15. Got the trusty old meter out and found this one to be 2.7uF - replaced with a temporary 47uF 50V cap and the display was as good as new.
Just placed an order for 5 of these with RS so I'll pop over and pick them up - hopefully I'll get a few more years out of my workshop amp (hooray - music again :-)
Thanks again bud
Dom