Xbox 360 Internet After Sales Problem

I purchased one of the above for my son in December 2008. Just before last Christmas it developed the Red Ring of Death problem. My son contacted Microsoft who asked him to send it to them free of charge, they would fix the problem and send it back to him. A few days later they notified him that they could not fix the problem as the console showed signs of being "tampered with", we know this is not the case, certainly whilst it has been in our ownership. I then asked the on-line company that we purchased the console from to respond to the fact that the console had been tampered with previously, if this hadn't happened during our ownership, were they selling re-conditioned units? We asked Microsoft what the signs of tampering were and they said that the identification mark on the motherboard did not correspond with the identification mark on the console itself. Well we are bemused, we know nothing has happened to the unit during our ownership, the company we purchased the unit from deny anything happening to it with them previous to our ownership and deny selling re-conditioned units. All I know is that we have now had to pay nearly £200 to replace the unit and want to get to the bottom of who has done something they should not have done prior to our ownership of the item.

Any help regarding this matter would be very much appreciated.

Thank you.