Have you advised local council you are doing this work ? if not,The only advice I can give you is, Don't on any account do it yourself, it is now illegal to do this as a DIY job, sorry to sound such a misery, but if you do change it yourself and something goes wrong you will be in big trouble,they are really clamping down on this type of work, call a sparky in and let him sort it,
The only way you could do it yourself , is if you get onto building control at the council and inform them, they will then somebody out to test it all, but tell them first. good luck with getting it done, Ray