Well done Malc, that's a really great valve tester, one of the best ever made. But yes, they do have very fragile meters, most of which meet their maker not due to abuse but from damp. The dreaded "green spot" corrosion attacks the very fine wire in the 37uA meter movement and takes it open-circuit. So tuck it up nice and dry at all times - no leaving it in the shed under a sack!
You can do a bit of electrical protection of the meter by putting a couple of Si diodes (one each way) across the terminals. Crude but effective.
And do use a bit of switch lubricant on the various switches. They are probably getting a bit dry by now.
If the two clear plastic skirts start to craze and crack, just use an Avo knob from any Avo equipment of the same age (I can supply if needed) - the smaller knob, non-skirted is still perfectly usable.
Start by testing low-value valves! (and watch out for leaving that heater volts switch set for P-series valves!).