[Herald1360]

I wouldn't go to electrolytics- you can get 630V 4.7uF polyester types for a few pounds and they're not overly large. The smoothing choke in the HT supply will compensate for the low C values and pentode output valves are tolerant of a fair amount of ripple on the anode supply. In your circuit it looks like the 1100pF (1000cm?) capacitor is the one that should be 1600V- 1000pF will be fine and easier to get, though you could use a couple of 2200pF 630V caps in series if you're keen on the 1100pF bit.