Vaillant Combi low hot water pressure.

partial blockage in cold water input filter ????

There is some sort of blockage. Frequently, this is due to scaling up of the heat exchanger. Do you have hard water? Do you have an ion-exchange softener, or a phosphate doser? (Ignore magnetic or electronic water "conditioners", they only work if you believe in the tooth fairy).

Other possibilities for blockages are inlet filters and partially closed isolation valves. The latter isn't particularly likely as you say the problem was gradual and progressive.

Christian.

Agreed there is something restricting the flow. The water is very soft - nothing visible in the kettle after over five years use! I think that it's time to take off the DHW heat exchanger and eliminate any blockage due to dirt (?). The isolation valve has been checked and is as clean as a whistle.

Thanks for all your suggestions Russell