[Arran]

Ed;
Before getting into the IF cans I try to make sure that the tube sockets, and tube pins are clean, those 7 and 9 pin miniatures are as bad as loctals for experiencing intermittent contact this way, particularly on cheap sockets. Incidentally this is part of why I wash most of my chassis down with a solution of TSP and hot water, it does a good job of not only removing dirt but oxidation, particularly from aluminum (or as you say aluminium) parts, it seems to prevent a lot of intermittent contact issues that way. Brand Z transformers do experience silver migration as well, but the construction is different from the Automatic style cans, and others, the mica sheets are not buried in the bases of the IF cans, and I believe that brand Z sometimes also gave the values of the caps, but maybe not this time.
Regards
Arran