[Arran]

 If that 375 mmf  mica capacitor C27, is really the problem, and it is buried, it may be easiest to remove the band switch assembly to get at it rather then trying to get a soldering iron between the coils.  If this is the case then it would be best to drill out the rivets holding the trimmer capacitor bank in place, and desolder the end grounded to the chassis, rather then to remove each wire going to the band switch assembly, then you can use some small machine screws, nuts and washers to reattach. Trimmer 19D should be easy enough to test on it's own by disconnecting one end, it's possible that it may be the culprit if some garbage like solder or a strand of steel wool got into it, leakage is not very likely since it uses a mica sheet.
Regards
Arran