Now 2 weeks on I am really getting the hang of this heating. During a 'sun/cloud' day the panels heat up less than on an all sun day - and the temperature in the tank doesn't get above 50 -55 - but this is plenty hot enough; an all cloud + rain day it may well be below 50 in the morning but has never been as low as this by the evening. Having the gas on for 1 hour is more than enough if a lot of hot water is needed early in the morning after a very cloudy day.
My gas usage for 2 weeks is 1.99 units - a reduction from 10-12 units in previous years; so a saving of 80%; if this holds up I can begin to believe the claim of a 40% over the year