Hi James and thanks for your input
You are, of course, quite right about the boiler overloading the supply, the engineer from Southern Electric said the same thing
How I do envy people like you and David who can do things for yourselves, the limit of my technical expertise is replacing a plug, so I have to pay a professional for every little repair/alteration and live within the limits of what they advise
So in March I bit the bullet, paid the plumbers to take out the remaining wet radiators, then paid Southern Electric to put in storage heaters upstairs, I do need the heating on somewhat earlier than the Flexiheat evening tariff drop at 7.30pm, or it is a very miserable business getting ready to go out in a perishing cold bathroom & bedroom!
The incredibly expensive boiler now runs only 1 hour a day at overnight rate to heat the water
The payback period will be quite a while, but the house is very much more comfortable and the running costs are coming down
The moral of this story is: when you buy a house, get the heating system checked out VERY thoroughly and don't rely on what the survey & previous owners tell you ...
regards, Mary