off the top of my head i cant be sure whether the pilot assembley is still available for the netaheat or not, i worked on a netaheat last summer and we had to order a new gas valve and ignition control box for it so some parts are definately available.
i would be wary of people saying the parts are obsolete and you need a new boiler however, there seems to be a lot of this going on, in some cases its down to young engineers who haven't been trained on older boilers and are scared of them and in some cases people are just trying to flog you a new boiler to make money.
its well worth finding out for yourself if the parts are available, you can try ringing time and temperature which is part of the plumbase group, spares and controls in sovereign rd kings norton birmingham or ring baxi potterton themselves, you should be able to get the phone numbers off google.
as for your new boiler, its probably going to have to be a condensing combi to comply with the energy efficiency laws. theres no such thing as a problem free combi, they are very complicated machines that need regular servicing and you will have to spend on spare parts now and again, think of it like the way you maintain your car.
the best combis you can buy are from worcester and vaillant, the worcester bosch greenstar being regarded by many as the very best but my boss rates vaillants as highly if not more. halsteads are also decent boilers.
as cleversod says you need to pick a boiler with enough power output to satisfy your demands, any combi can power the heating but to get a good hot water flow you need a powerful combi. 28kw is probably the minimum power output you should go for and cleversods 37kw would give great water output.
beware of installers reccomending a particular boiler, it could be because they are getting a good discount on that paticular boiler from the plumbers merchants.
please let us now the outcome.