[k9uw]

I guess (purely out of interest) one way to find out whether they are mica or not is to measure them. It might be a bit tricky with a regular meter as it will show about 100pF when not connected so it needs too be zeroed first; if it shows close to the nominal value, it is then a mica; if it shows a different value (paper caps migrate as fas as 2x to more the value) then it is paper.
Has to measured out of circuit, with the wires around clipped.

Good suggestion, Morzh!  It took a while to isolate the leads from one of the caps inside the Bakelite block but, after adjusting the meter to compensate for capacitance in the leads, the cap that had been 110 pf had jumped in value to 245 pf.  They must be paper, and they're coming out!