I don't know it all, but I can have an educated guess. It sounds like the diverter valve p/no 252457, is sitciking. Very easy diy change.
The vessel is an expansion vessel, when the system is drained, pump this up to 10 psi or 0.8 bar. You can also remove and clean the plate heatexchanger at the same time. Clean and grease the o rings when refitting. Another service item worth doing is the rubber hose (to the left of the main heat exchanger) and the pipes, get blocked on these and a worth removing and cleaning. Where the pipe joins the t piece at the bottom is a filter. This blocks and gives hot water problems, easily removed with a bottle cleanind brush. All in all should cost around £100 in parts and two hours of your time.
One more important thing, are the nuts on the main heat exchanger Nyloc type? It appears to have some heat damage and may need a new seal. This is not DIY I'm afraid as combustion checks need to be done afterwards.
At my prices (London), you could expect to pay no more than £220. You could call vaillant for instant cover @around £300 including all parts & 12 months warranty. I dont cover Wales - but it is definitely not a throw away boiler. I have a restyled version of yours and I'm expecting 15 years plus out of mine.