I have just moved a small (1.7kW) storage heater to a temporary location
(next to my desk!) where I plan on using it until March-ish. We are on E7,
and downstairs (where the heater now is) has separate fuseboards to
upstairs. I could wire it into the off peak fuseboard (which feeds just the
lounge storage heater), but it would be easier for me to plug it into the
mains (on a timer) on account of its temporary status.

Other items that run off the downstairs mains overnight are the immersion,
the washing machine and the tumble dryer. I tend to have these come on
about 5am so they're all nice and fresh for me in the morning. I imagine
the storage heater would have done most of its warming up by then and would
be cycling on and off according to its thermostat.

On balance, would it be ok to plug it into the mains, or do I really want to
be hard wiring a new spur into the off-peak board?

Many thanks,
xena