John, l had a terrible hum problem which was resolved by replacing a phenolic valve mount, it had suffered a lot of heat and become conductive.
Looking at the service sheets it appears that C20/C21 are held by a P-clip. I would have lifted the two positive legs and used croc clips to temporarily insert two new ones.
Isn't hindsight marvellous! 20/20 vision!
Hopefully I'll see you in November...