In reply to A Game of Chance:
Turns out it was the MTU value on the router. I've no idea why the value wasn't the correct one, I certainly haven't changed any router settings recently and the value wasn't the default that the router documentation states.
All back and running, bizarrely with a download speed that is well in excess of what our line is supposedly capable of - we are a long distance from the exchange so don't expect high speeds.
ALC