I found the PC running slow and initially found that Sophos was taking a lot of resource so removed that. But it was still running slow and freezing for 30-60 seconds at times. I then thought it may be due to trying to in some way get to the university's domain so I changed it from domain to workgroup and then rebooted.
Oh dear, oh dear, oh dear.
I couldn't login, password rejected. Tried various things, no joy. I then realised that perhaps the user credentials must in some way have been on the University's servers - though quite how the login worked when it was off-line I don't know.
Googling around I found quite a few hits on the problem, with few solutions. For a few minutes I thought I'd messed it up well and truly. Then one hit referred to Hirens...
So booted up and ran the password utility on Hirens. What a useful little tool . Was able to recover and reset passwords so all is back in working order now. And it's running much better.
Thank you Hirens