Red warning light mean that the overheat stat had failed. Its the one that screws to a square plate on a pipe which is to the right of the boiler. Part about £10
If your pressure is all over the place you need to repressurise the expansion vessel. Just use a bike pump when the system is drained to pump it up to 14 psi (1bar). If you want to test your expansion vessel find the car valve on it and as if your letting a car tyre down let the air out. If water comes out youve been unlucky and the daiphram has punctured. Other wise its a free fix.
If water leaks from the boiler get the plumber back in to put it together properly, as he should of done that in the first place, thats just sloppy workmanship.
If the water heats then cuts out that means that the heat exchanger (water side) needs cleaning. Use wonder wheels (the stuff for your car). The exhanger need removing to do this. It will silt up again if you dont clean your heating system out and use a water stabiliser in the radiators. Fix for free if you do the work yourself. Takes about an hour to do but becareful attaching the exchanger to the diverter valve, its easy to cross thread.
These problems are all down to people not servicing the equipment each year or plumbers too busy to do a proper job when there is a problem. Plus most plumbers are parts changers not technicians. They dont think about the problem and go for what they see as the easiest fix. Most problems with this boiler can be fixed for under £20 in parts.
As I write this my boiler has now been running for years without problem. I have serviced it and had to change the diaphram once apart from that they are an OK boiler. There are better out there but you get what you pay for.