Water Heating Options?

[Clodbuster]

My Gas combi-heater is giving hassles and advice is to replace it. It is 19 years old, Never been that pleased with it and bath water never been sharp plus annual charges etc. (The only solution to that is apparently to run the water in slower which is actually the same effect) The central heating side is good.
Is there anyone with experience of air source heaters that will give their opinion and possibly any other solution.
Thanks.