A brand new Dell inspiron 15 is now only £349 including VAT, shipping, etc.
Depending on the spec of your old laptop, if this one is an improvement, I'd go with the ne one, which will net out costing you less than £250, plus you get free Windows 7 which is an absolute must (been running it for 6 months now - vast improvement over Vista, quicker, and better than XP).
However if, like me, you bought one that was top dog for its day and spent about £1500 for the greatest displays, CPUs etc, you may well find your old laptop will still outperform the standard new ones, in which case do the battery/disk.
Also be wary of aftermarket batteries. I've had two in the last 6 months purchased for a thinkpad that simply won't charge after about a week of use. I've also had capacity issues with others where they simply don't do what they claim.
BW