Blacksheep, I too used to work on VC 10s at Brize, but unlike you have had an experience where condesation cause repeated failures in the autopilot system, mainly lat and long amps. We were playing around in Malaysia with the tankers doing 4 or 5 hour sectors. Then on the ground overnight. Come the following morning during crew-in, AP would fail to engage. Replacement on long or lat amps would fix it, but when taking those bloody heavy boxes out from the radio racks, there was water coming out them! Relative humidity was 100% and it was monsoon season and very wet and hot. We later found that the main cause of this problem was the use of airconditioning carts plugged into the AC system. This must have cause the radio rack cooling to get too cold and consequently condensation when the cart was removed and the aircraft downpowered for the night.
On the subject of dehumidifying aircraft, my airline is to trail a system on our B757s and A320s. 757s are notorious for "plane rain" and the usual cause for TCAS and enhanced GPWS failures due to moisture ingress in the antenna connectors. However this is not the motivating factor to trail this mod. The expected result is weight saving of over 500kgs of water saturated in the insulation.