Just to answer my own question as I've finally sorted out the problem. It would seem that the fan finally died. Somehow during a previous clean out for overheating I managed to crimp a wire for the fan with a chassis screw. I've tried rewiring but this may have been the original cause of overheating in the first place and not my crimping. I was sure I could hear the fans whirring away but when all was opened up..no fan. The down side is that to replace the fan is more than the unit is worth!