We need some help after having a shower installed by a builder who did a runner before completing the job a few years ago, it all works and the pressure is OK but the heat of the water is terrible even on full heat. It used to be working fine but for some reason the heat is gradually going. The hot taps around the house are mega hot as we have turned the boiler up to the max but we can't seem to solve the problem. It does get a little hotter when other cold taps are on but is a little dangerous for my liking!
Something you could check first is filters. If the hot inlet has a filter which is blocked, the cold could be taking priority, causing a reduction in outlet temp. Also look out for damaged or missing flow restrictors- refer to manufacturer's instructions.
If the filters etc are ok, then it's obviously something else causing your trouble.
From your post, I'm assuming you have a combi boiler, both inlets to the shower are balanced as it's worked previously and that you haven't recently made any changes to the boiler or water pipework in your house and that no road works have taken place that have affected the incoming mains pressure.
Is the shower thermostatic? If so, it either requires adjusting, or a new thermostat cartridge.
Try contacting Bristan- 0844 7016273, ask for the technical department (I don't know for sure that there is one, I've never rang it, but there's bound to be somebody who can help troubleshoot the fault). If not, you can email them directly from their website.
Tell them your shower model and its symptoms, including there has been no recent work carried out inside your property or on waterways in your street and that you have cleaned the filters and the water temperature from all other hot outlets is ok.
I'm sure they will tell you it's the thermostatic cartridge at fault and will be able to either sell you another one, which will be posted to you, or you will be able to get a part number off them and search for it on a shower spares website, such as showerdoc.com (who's services I've used before and recommend).
Bristan do offer 5 years warranty on shower mixers, so try your luck getting a part for free if you have proof of purchase or if the product was registered when fitted.