I had a bath!
I monitored the boiler while I was running the bath. Readings as follows:
d.36 initially 10 l/min settling to 9.7 ... 9.6
at intervals
d.40 55, 74, 76, 78, 77, 78, 79
d.41 45, 54, 56, 57, 56, 56, 55
Water ran hot the whole time.
One thing that dawned on me - I guess I knew...