Combi boiler heating water red hot for first few minutes

Without the benefit of seeing the property to carry out a heat loss calculation which your installer should have done before selecting the appropriate size of boiler, I can only speculate that the boiler does seem a little bit over sized. You state the DHW rises when you initially call for hot water but then drops to a more comfortable temperature after several minutes, this clearly is not delivering water a the required pre-set temp. Suggest because the boiler is less than 1 year old and still covered by the manufacturer's warranty, you call them and request a technician attend to rectify this fault. The boiler should have a Domestic Hot Water NTC Sensor fitted, this maintains the temp you have set it at the control panel and should deliver that desired temp until you adjust it. Soon as you turn a DHW tap/outlet on the burner will light automatically and the temperature of the domestic hot water is controlled by the above sensor.

We used to find that the silicone tube off the gas valves was always too long which stops the boiler from modulating and always got trapped when the front fascia was refitted. When it fires up it goes straight to high flame and stops there?