Be wary here- the energy supplier 'engineer' may turn tail and flee when he sees that bit of MI. I'd suggest buying 3m tails before he turns up (get them from somewhere that does returns in case you get lucky and he has some on the van) and be prepared for new tails in the existing CU.Thanks mattylad, but I've just checked with my energy supplier (Bulb.co.uk) and they will come and fit an isolator switch for £120 or better still, only £60 if I agree to have my meters (gas & electric) swapped for smart meters at the same time.
I have so far resisted changing to smart meters but this seems like a good deal. Once the new isolator switch is in, it will be much simpler to connect the new consumer unit using ordinary tails.