Sussed it out eventually. It all came to light when I tried to do a Hardware test and nothing happened, so I printed out the page 34 and tried again from a mains off boot up, still nothing. All this time I have been using a wireless keyboard and mouse, so I went into the loft to search for the USB keyboard the iMac came with and started again tried to do a hardware test again, no test came up but it did go into the proper Apple bootup. Replaced the wireless equipment and all still works as normal.
No idea why the wireless keyboard gave the problem, this is for Apple to sort out, not me I'm happy all is OK.
As its working I'm not going to try and fix it
No idea why the wireless keyboard gave the problem, this is for Apple to sort out, not me I'm happy all is OK.
As its working I'm not going to try and fix it