I've just bought an Invac 250w modified sine wave invertor for just under £85 via Cruisermart which runs off a cigarette lighter plug , but beware it has a female "kettle" type plug & can I find a male plug adapter ? Nope !
Other posts show competitive prices at retail. If you can't deal with R.S., who are trade, call Maplin Electronics who's head office is in Southend. (Branches in major cities). They have Invertors and will also have the male IEC plugs or leads that Hamish needs.
I just bought a proper 12Volt adaptor for my laptop, which will be more efficient than an invertor and good in the car too. £69.