I've just replaced the EAS compressor!
I can't find a leak in or near the valve block/dryer but according to the nanocom the exhaust valve does seem to be open then shut quite a lot even when the compressor is running. How should it be behaving?
For the record, the compressor filled the reservoir in about 6 minutes and then shut off, so I'm confident that bit is OK. When I shut the boot and let it lift to motorway height the compressor kept running for another 5 minutes before I shut it off. I guess there's either a leak to or in a bag or the valve block is doing stupid things. I'll have to do some grovelling in the snow and see what's up.
Let's hope Marty's compressor didn't die the comprehensive death of its predecessor and I can use my new rebuild kit to provide a spare!