Sorry to post yet another 'new boiler' question. Hoping someone can help.

As part of a loft conversion, we replaced the old boiler system.

The old was a 'traditional' open vented boiler system with a pump and a hot
water tank, a salamander shower pump and a Bensham thermostatically
controlled shower mixer. The new is a Worcester Bosch (WB) 28CDi RSF combi
in the loft.

Since installation (by a plumber working for the building contractor) the
heating has been fine, but the 'hot' water has been tepid.
The plumber went through the trouble-shooting routine, said that a flow
sensor was defective and called WB. Today the WB guy came, tested
everything.
He says the sensor is fine but that we had a 'cross'leak'. He thinks that
the shower mixer is allowing cold water up the hot side and into the taps
(which he tested by switching off the hot supply and getting a good flow of
cold water out of the bath hot tap).

The local bathroom place says this is a good quality mixer suitable for
mains pressure and disputes the diagnosis. Any thoughts please - just trying
to make sure i'm not getting dicked around and looking forward to a hot
shower again.

Called to a similar problem the other day - shower about a year old - boiler fine - plumber changed the seals and fitted a pressure reducer in the cold feed (he said he measured 7 bar incoming) - customer happy.

Jeez, another make I have never heard of, it's as bad as boilers, dont tell me they have been making "qaulity" showers since the ice age and supply to the whole universe, thats why we have never heard of them  ???