I am not doing anything too drastic just yet. I have discovered I can stand it upright and this may help the cooling. However I am not really convinced the problem is caused by overheating.
The problem (happens once a day or so) is certainly solved by switching off for 5 mins and then on again.
Maybe it is something to do with the upgrading of my BT Broadband from the old 1.8Mbps/0.2Mbps to (big deal!) 2.7Mbps/0.4Mbps ?