So I've checked and the cable is 6mm throughout
Ok so i think i know what i need to ask for, time to choose a shower!
Anything to watch out for?
Also there was mention that the shower needs to be moved to the RCD side, on a side note, what sort of appliances generally go on that side?
Are you sure you feel competent to change and/or move the MCB around the Consumer Unit - don't forget as well as the MCB you will need to change the position of the neutral and cpc conductors and the RCD itself - make sure the circuit is dead before proceeding.
You will also need to remove and make safe that light/shaver socket fitting, if that is what it is, next to the shower.
Finally, you have what looks like a Mira electric shower - it will probably work out cheaper to have one their engineers come out to look at shower and repair it.