I suggest you get someone in to have a look, There is possibly an alternative route which has been overlooked.... and even if not, if one goes into a job like this on a Saturday morning when the office workers are all enjoying a lie in, then these things don't normally take too long.
12" Masonry wall is pretty standard... my 'normal' set of drill bits for getting through walls that lives in the box with the drill are all 18" long.... I also have 1M bits, but I can count on one hand the amount of times I have needed them!
Thanks. It looks like we either limit ourselves to the 3.1kW or add cable which might not be as scary as I'd thought. Luckily a colleague has a friend coming to visit who's a sparky so we'll see if he can take a look for us.