Why when the shower is used is there a nasty fishy smell coming from the cupboard behind where the shower is located?

Its caused by dampness. The only solution to this is to strip your shower completely, tiles and old waterproofing (if any) and re-waterproof the shower and retile. I also recommend changing the shower trap and waste pipes along with the shower water pipes if one can afford to do so. Rather do the job once properly.