Thanks for the suggestions. All a bit beyond my knowledge but hopefully tried them correctly. Taking them in turn:
It's not a home setup - the hosting company is Krystal Hosting in London and it's on a shared Server IP.
nslookup points correctly to the new host for my website and is the same on all three PC's.
I can ping the IP address on the PC in question.
I can tracert to it on the same PC - 15 steps.
Entering the IP address into Firefox instead of the web address gives the following - same on all three PC's - two of which are working correctly with my website :
