Is it a mixer shower connected to the bath taps, or a separate electric one? Did the shower have a 12 month guarantee?
From the Sale and Supply of Goods aspect, your contract is always with the retailer, in this case MFI, and therefore it is they who are your first port of call. Legally, where a fault ocurrs in the first 6 months, the retailer has to prove it was not present when sold to you, and thereafter the onus is on the customer to prove the goods must have been defective from first use. In your instance, you can demonstrate that the goods had defects very early on, and this will help your case in my view.
When purchasing any new goods, you have a right to expect them to be free from defects, and dependent on when any fault may later occur, you can expect the retailer to either replace the item - usually if less than 28 days old, or offer to repair the goods - bringing up to a satisfactory standard within a reasonable time scale. If they cannot repair, then again dependent on age, you are entitled to a full or in some cases a partial refund. The later is usually when the goods are almost out, or are out of the initial 12 month guarantee period. The partial refund reflects the fact that the customer has "enjoyed" the use of the item for a period of time, though in your case that word might be "endured"! Sorry can't be more specific, but in consumer law, there are always Grey areas such as wear and tear, fair use etc. If it were my shower, I would go back to MFI, explain the history of the problem, and if possible give them dates of when you had problems, and when new parts were supplied. Be polite, and ask them what they are prepared to do to remedy the problem. Do not accept an answer such as "it is not their problem, but the manufacturers". You bought it from the retailer MFI, and that is where the buck stops. Hope this helps a bit.