Before you go any further check the gas rate with the hot water all on full
and compare with the specification for the boiler. Should be recorded on
the Benchmark.

I can't see a gas flow rate (or anything in m^3/h) on the Benchmark or
the Eurotron "receipt". The tech specs say it should be 4.2 m^3/h.
I've got a Corgi plumber (not the same one) coming around on Monday.
Is it a quick & easy test for him to perform if I give him advance
warning tomorrow?

I know that they did test the gas as Transco came out to replace the ?
regulator? (or something similar on the meter side - I remember some
explanation about how otherwise there was a chance that when the
boiler came on the gas to the hob might cut out).

Its possible that the DHW cold feed has been restricted
and the gas rate is limiting. With hot water on full what is the indicated
temperature for the hot water?

It rose to 62* after running it for a minute or so.

Thanks