Thanks everyone for their help, I finally worked this one out.
After checking everything in the reset section and where the very small amount of corrosion was I decided to have a look at the chips that I hadn't touched.
I reseated U2 and swapped the PIA chips.
U10 had a 6820 in it and U11 had a 6821 in it.
From memory I reckon I changed U11 for him when he first got the machine so I was confident it was ok (it was new when I installed it).
When I started the machine, it started perfectly so then I wasn't sure if it was the reseating or the swapping of the PIA's, so I swapped them back, sure enough the endless re-booting.
I also tried another PIA in U10 and the machine started fine.
So it looks like the MPU was a bit fussy about the 6820 in U10, funny though it had been running fine for at least the last year.