All sorted. I shot down to DSE in my lunch break but the unit they had didn't look very heavy duty. It had a big warning about being suitable for testing only and I don't know enough about them to be sure it was suitable.
So I popped in to Battery world, they hooked me up with a 12v battery and a female cigarette lighter lead. I can plug in the lead that came with the scope and run it from that, then recharge in the morning.
So for anyone else considering this scope - be aware you need to sort out a power source first otherwise you won't be able to tinker the night you set it up
Now... just to get rid of these clouds...