Does anyone know anything about gas heating/water boilers?

[Toodledoodle]

We have a gas boiler that heats the water and the radiators.

We've had an on/off problem for a couple of months now where the hot water (not heating) goes off. When you turn on the hot tap it starts out cold and gets colder (absolutely freezing). It works for a while, then cuts out, freezes you in the shower then the water goes hot again. The problem is intermittent, so sometimes its fine for days, weeks, then 'goes' again. It doesn't specifically happen at certain times, or of the heating on/off, so there's no pattern that we can see. The pressure is fine all the time. There isn't a flame we can access.

I really don't have the money for a new boiler and have just had to shell out £££ for some other maintenance recently, so not too keen to get the gas man out either if there's something I can try myself!

Does anyone have any good ideas I can try?