I have a nasty feeling that the warranty only covers the building itself against structural defects, not the internals, but well spotted Dan, definitely worth contacting the builder.
Okay, so we know it's a thermostatic shower, so we are in fact back to a possible filter problem inside the thermostat, but it's more likely to pick up crud in an older property. As it's a mains pressure system for both hot and cold water, I doubt if you'll find a pressure reducing valve anywhere, but being a new build, there might be a temperature reducing valve that's Donald ducked, but as it's a thermostatic shower, a temperature reducing valve is a bit redundant - but there's a lot of duff plumbers around.
Have you checked the operation of the thermostat on the shower itself, and adjusted the temperature up and down. if you turn it cold, do you get a good flow of cold water, or turn it hot and get a good flow of hot. This would show you whether it's the hot or cold that blocked, but if both are, then it's the shower unit itself that's gone.
Either way, unless you feel confident to start taking pipes apart, this is the point to ask your neighbours if they know a good plumber. Best of luck