try running the apple hardware test, the following article tells how to do that:
also this article tells how to troubleshoot the gray screen issue:
you can skip the first 2 steps in this article and move on to the one where you repair disk
Thanks for the quick response. I already did what I could re the article on troubleshooting (although I found some of it hard to follow) so I did the test - result was "no trouble found" so I hit "restart" hoping it would magically fix itself, but I'm back to square one. Grey screen, then black screen. Should I pay the £35 and hope the guy on the end of the phone can help?