Well, works now :-)

After getting to almost grumpy state due to lack of response from the computer company re when the engineer might arrive, he finally arrived this afternoon. As I suspected something had come adrift. In fact the whole CPU fan heatsink unit had come loose from the motherboard. Boot up, CPU rapidly gets to hot, so machine was turning off. All seems to be working fine now. Now plenty of scope for diversionary activity getting everything transferred, installed etc.