Update: I have been looking on forums and seen that with difflock engaged the car will make a knocking sounds whilst turning. (note - the dash light is not on and the selector was in normal H). To test I jacked up one wheel of the car, put the lever into neutral and tried to spin the jacked wheel, it would turn slightly but then stop.
Based on this it was clear the car was stuck in difflock.
I called the original repair garage and told them the problem, they suggested I brought the car back in. I refused to do this as Durham is over 40 miles away. I suggested he picked it up with his lorry, however he decided (rather wisely) it would be best for me to take it to a local garage to fix the problem and sort the bill out for the work from there.
Hopefully it will be sorted...