hold off may be a fart in the wind here but depending on the make and model of that combi you may still be able to get parts, im a gas engineer and you wouldnt believe what you can fix, its finding an engineer that will spend the time looking for the parts! Being a combi failing on hot water i would say it could be as simple as a split diaphragm which is very common! 16 years is old for a boiler but you could still see 20 out of it! you'd be amazed what you see on a daily basis! The issues that can lie when getting a private installer to fit the boiler is making sure its fitted to current standards, standards do change but not to a massive extent so if you were to get anther engineer in say from british gas they could categorise it as at risk if something as simple as the flue not being sealed internally/externally which would then have to be turned off! Just because an engineers is gas safe registerd doesnt mean he will meet standards and go by the book, most dont even know what alot of the new standards are! So just do your research, if you stick to the way manufactures instructions say you cant go wrong!!!