I fit mostly Vaillants and service
a lot of different boilers. I would not have known that this was a stock
fault with a free(?) upgrade part (non of the ones I look after have
shown this problem yet [1]).

[1] Partly because an annual service for these would usually include
sniffing the flue gases and cleaning out the condensate trap.

After the Cowboy's visit, I contacted Vaillant and they sent out one
of their own engineers to do the job properly and he advised that
there was a general problem with the condensate arrangement. Hence the
frre upgrade.