Many thanks Karl.
When I contacted TT for advice I spent almost 2 hours with their operative running through all sorts of checks, despite saying that I felt sure it was something that could be changed in thei system. In the end the operator insisted it would be necessary for a bight sparks engineer to visit my home, and I eventuall gave in.
I will now cancel that appointment, so I am grateful that you have been able to change the box for me, but do wish TT would inform their staff that this can be done quite easily, without an engineers visit.
So thank you again, and keep up the good work !
Clive