That guarantee is fairly standard, serviceable parts would not be covered as they are expected to wear, however most manufacturers are sympathetic if something goes wrong within what might be considered an unreasonable time, standard taps don't have a lot to go wrong anyway
Just replace the top outlet with a surrey flange, you may need to do a bit of pipework depending on how much room you have above the cylinder but thats more preferable than trying to fit an essex flange
The problem with trv's is that most people don't set them properly, turn the ones down that get the hottest quickest to balance the system, if you are not going to have a trv free radiator you should have a bypass, normally the bathroom rad is left without a trv
I have a problem with a main combi boiler in that the hot tap has to be turned on full for a time before the boiler kicks in, it used to come on instantly regardless of flow, any ideas??
Thanks