Slowly trying to recover my PC.
Replaced (as in, put it back in) the third RAM stick this morning. Switched everything on. Nothing.
Took that stick out again. Switched it on. Works.
I'll try that 3rd stick again later. Just creep up on the machine when it's not looking. If I take it by surprise it may not notice.
I may have put it in badly, so I'll try again