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?

Definitely off, if it is for that period. If it is just unused overnight,
savings will be minimal. However, over the period of a week, you'll find it
cheaper to only heat weekends.

However, if it is electric, you may find it takes an age to reheat, so will
need to do some calculations to determine when the timer should come on.
With gas, it should only take an hour or so from cold, assuming a modern
system.

Christian.