I have brought a new shower & the sparky is wiring it tonight & has asked me to get behind the tiles! Help!?

you do not fit any type of junction box into a wall ..how can you get to it for maintenance? ..but most important ..if he is not nic reg then he cannot do it ..and i doubt if he is or he will know you must fit the appropriate size cable directly back to consumer unit ..and fit a mcb ...you should all so get a certificate afterwards ..im a builder ..would never ever connect to an old black rubber cable ..you should all so check if your earthing system is up to scratch