Looking at the manual for
http://www.creda-showers.co.uk/shower-range/creda_electric_showers/creda_family_showering_range/creda_aquawave_electric_showerSuggests that the operating water pressure range is between 0.7 and 10 bar. It also says there is a temp cut out for if it goes below 0.7 bar.
Its very unlikely your pressure has gone above 10 bar.
Looking about the internet suggests mains pressure should be around 2 to 5 bar, with a minimum of 1 bar.
http://www.heatweb.com/techtips/mains.htmlDo you have low water pressure in your house? or does it fluctuate?
That manual says this safety cut out is goverened by a pressure switch. It is perhaps possible that this switch has become faulty and wont acknowledge a higher pressure and therefore wont heat the water.
Contact united utilities and ask for some maximum and minimum water pressure data in your area. Then contact the manufacturer and tell them that your water supplier is good for pressure and that If they suspect it is a pressure fluctuation, and UU say the pressure is good, then you suspect the pressure safety switch is faulty and they should replace it under thre warranty.
But the water pressure safety switch may not be the problem. It could be another faulty part. But if you can tell them that your water pressure is good then they cant come back to you on that.