OK, managed to scrounge some fuses from stores at work. I put one in and took all the power valves out, and powered up. The fuse was fine. I put the valves back in and turned on, when I turned on the standy swith I got a loud pop from the speakers (even though the volume was at 0) and then crackling and rustling. The fuse did noy blow this time, but the bias voltage measured off the scale on a 200mV range, even with the pot fully down. I swapped back to the older microphonic valves and now the amp is fine, apart from the slight rumble from the dodgy valve(s).
So, it seems that my unused power valves are buggered (what are the chances of that?!), which is rather annoying having kept them specifically for this eventuality, so it is a call to Hotrox tomorrow to get some winged Svetlana EL34s.