Morning @kezo Many thanks for all the information, some of which I understood ? I’m going to contact my local electrician tomorrow and ask him to call around and answer the following questions;
1) Are the two consumer units supplied from a split cable from one set of incoming tails?
2) Does the consumer box in the extension have 25mm2 tails supplying it? Cost to achieve this if not.
3) Is the garage supply coming via an MCB (fuse) from one of the consumer units?
4) Is the garage electrical supply 10mm2 cable (or even 6mm2 cables)?
Does that cover everything?
Final question, I didn’t understand the point about they could refuse an install if the cur-out fuse needs to be upgraded to 80A or 100A. Is that a big / complicated job? Cheers.