Thanks both of you for your advice.
I have been toying with the idea of replacing my old laptop for months but a New Year's resolution was to spend more time doing and less time putting things off, so I took the plunge and ordered an HP i5 with 8GB RAM and 1.5TB HDD, Intel graphics, for just shy of £400inc [1]
It's from PC World which is a shame but from what I can tell it should handle Ubuntu pretty well, with 1.5TB giving me plenty of space to try alternatives too if I feel like it. Even worse it's blue, but that's probably why it hasn't sold out like the white version, and I'm really not that bothered about the case colour! I do of-course have some time under distance selling regs to change my mind so if anyone thinks I've made a terrible mistake please chirp up (within 7 days :-)
[1] http://www.pcworld.co.uk/gbuk/laptops-netbooks/laptops/laptops/hp-pavilion-1...