F4 is for the solenoids and wont affect boot up...

No 2nd flash suggest problems with the 6810 RAM.. make sure its seated in its socket properly before doing anything complicated

Thanks, Retropin. OK, some progress. I'm not sure if it was the pressing of the chips which did it, but I now have six flashes on the mpu led. One flash, then a pause and five more. It doesn't happen every time I power up though, sometimes I have to press the reset button once or more before it will flash.

Actually, I think it was doing this before every now and again.

Does this help?