Thanks for all the replies and suggestions, it's working, son is happy, end of story......
.....OK 'puter geeks, I know you're itching to find out what it was, so here goes. For those who are not interested skip two paragraphs...
After much prodding and poking we (Son & I) decided that the motherboard may be faulty and proceded to prepare it for return to the supplier. Asus' website was no use at all except for the insistance that the CPU connector (skt 775) protector was replaced before shipment. So, the graphics card, memory, and fan were removed, then the CPU was carefully packed away. At the point of fitting the protector it just didn't seem to fit snugly. and after several gentle tries at getting it to fit I re-examined the socket area. A small piece of protector plastic was seen to be occupying a corner of the CPU socket, how and when it got there is unknown, the CPU had not been removed before for fear of damaging it, on first fitting it had slotted in fine, and there was no way it could have been fitted in the wrong orientation. The plastic could have broken off on first removal, or during my attempts to repackage the mobo, or indeed it could not be significant at all.
Suffice to say we agreed to give it one last shot, during which we remounted the CPU with fan, reinstalled the memory, and slotted in the graphics card. Once the PSU, monitor, keyboard, and mouse connections were made, fingers were crossed and the PSU switched on. Immediately the monitor burst into lovely life - this was further than ever before, and we set up the BIOS and loaded the OS.
Now it works a treat and Son doesn't give me that "you're useless" look anymore.
Fathertobe, have you lost the tobe yet?
Thanks geeks