Pete: Many thanks, Indeed a small ceramic cap, perhaps 6 mm diameter and 2 mm thick.
Three are used for peaking the optocoupler response, across the -1.8 kV HV to the CRT grid and cathode.
They look fine your point of cleaning is great, as they are connected to transistor bases and the PCB is somewhat exposed under cooling grille holes in the cabinet.
Other Hameg repairs have mentioned failed caps
I will try some isopropyl alchol later on.