performa 28i -water in gauge, losing pressure, hot water not hot enough?

4 yrold combi boiler, no leaks in system, had a couple of people to look at it who have tried to rip me off at huge expense but now problem is worse and when on hot water mode, the central heating comes on though just opened the case and now this isn't happening, is it worth mending or do i need a new one after just 4 years, also a recommendation for a good competent engineer in worthing west sussex wouldn't go amiss.in case relevant only serves small run of 7 rads and hot taps in kit and bath.