Opal shower?

Hi I have an Opal 8-7 - 9.5KW shower which has completely broken and I know it is cheaper to replace. Not sure which one I need to go for which is likely to have same fittings etc. Also (I'm a single mum) how do I know whether my electrician that I used to fit this one would have put in 10mm wide cable because I am reading that I need 10mm cabling if I have a 9.5 kw shower fitted. Is it literally the diamet of the cable and is it easy to locate and measure inside the unit which I can get into quite easily. This is driving me mad we just want a shower! Also do i get an electrician or plumber to fit it? Thanks