Adrian C expressed precisely :
Initially connecting it up, it was was drawing about 100mA. After 5 hours
it's now drawing 400mA. Must have some 'inteligence' in it.

That is the usual behaviour of a completely flat lead acid battery,
rather than any built in intelligence. My (not RAC) unit has an
automatic charger built in, which cuts it off once the fully charged
voltage is reached, but yours might not do that (look for an LED marked
charged)- so it might need to be carefully matched in voltage output.
It lights up as soon as powered, then goes out once fully charged and
stays out until powed down and up again.

It's charger suggests it is 18v 0.75amp DC. If yours lacks the charge
control built in, you need a voltage controlled PSU of around 13.8 to
14.2v.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk