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Thanks all, most helpful - yes, we are asked to add the new channels every so often, so we do that as we go along.
I have just found something in our local paper - apparently we have two transmitters on opposite sides of the town and it's only one of them which is switching on Wednesday (the other's in 2012) so I need to check that out first.
All help, as ever, is very much appreciated - they've said not to do anything until after 6 am on Weds anyway. I may come back to you ;-)