TonyK wrote:
Hi

Just looking for a guide, not actual calcs, from someone who can give an
authoratative "yes" or "no"...

Fitted a 330l pressurised cylinder (insulated) earlier this year and to date
has been left switched on all the time due to high demand for hot water
literally 24hrs a day. Now demand is dropping off dramatically and will only
be needed Fri-Sun so...

Should the cylinder be switched off Monday - Fri am allowing the water to
cool and reheat from what I asume would be almost cold on Friday pm. Or, is
it more energy efficient to leave it on and let the thormostat regulate
power consumption to keep it up to temp throughout the week?

As others have said, it's best to turn it off.
However, if it's in a place where this is possible, you'll get a significant
increase in time to cool down if you get a 5 quid roll of loft insulation and
pack it (loosely, but leaving no air gaps) round the cylinder.