Vaillant Turbomax 828 problem?

This is a strage one! I am not sure whether my diverter valve is on the way out or whether something else is causing it not to work propertly. For last 4/5 years boiler has always been noisy when switching from water demand to CH and vice versa. Problem is as follows.

when the heating is working properly, if i turn on hot water for more than a few seconds, when i turn off the tap, boiler seems to go back to CH mode. But, it doesn't seem to be heating all of the CH water - the temperature gauge will go upto the relevant CH temperature at a rate of about 2 degrees per second (normally much slower whn working properly), and the pump seems to circulate the existing water in the system, and possibly also some of the water it has jsut heated. So gradually, the water being circulated gets colder, but the boiler keeps heating up some water, but i do not think that the full CH flow is finding its way to the heat exchanger. At the moment I can solve the problem by waiting until the boiler gos into CH mode and then turn the hot tap on for a few seconds. When i turn it off the boiler seems to revert fully back to CH mode (whereas running the hot tap for any length of time cuases the earlier issue). Wonder if the diverter valve is sticking or not working properly. Hope this makes sense, and wonfer if someone can help.

Is it worth joining British Gas homecare etc. I understand that you cannot clam within 21 days but i could probably manage longer ithan that, and claim i had just discovered the problem