Stu,
IMHO go for the Dell. I've bought them on behalf of company's I've worked for, for friends and family and have had no probs with them at all. Good, well built machines which seem to go on and on. They have a good after sales set-up. Using the laptops serial number you can go onto the Dell website and see what upgrades (part numbers, costs, etc) are available simply by putting the number in. They keep an online history of every machine they make which you can access, which is reassuring.
You may want to spend a few extra £ for an on-site warrenty, it beats the pants off a return to base warrenty. Nothing worse than having to send your machine back if, heaven forbid, something goes wrong with it. IBM have just sold their PC/Laptop business to a far eastern company. I'm not saying it will have an effect, but you could struggle a bit if they're going through business hand-over problems. You don't want to have to argue with someone that your machine is under warranty because your details got lost in the system. Dell's a well established company with well established processes and procedures which should act as peace of mind if you do experience any problems with your machine.
In terms of the spec - buy as much memory as you can afford - 512mb is good, more is better. It's worth getting it up front if you can afford it.
40gb hdd - see whether you can get an upgrade for this. Bear in mind that you'll use 25-50% of this disk space loading up your applications - albeit this is depends on what apps you'll be running. This could leave with with 20gb for your personal data. Nowadays iPods are capable of storing 60gb of music/video/photos, and you'll be amazed how quickly you use this up. I've solved this problem by buying a small (in physical size) 80gb external disk drive which plugs into a USB port. This way I can carry/move all my multimedia stuff without being tied to a particular hard drive in a PC or laptop. These are about £80-£100.
I'm sure they'll be others out there who have experienced horror stories with Dells and think IBMs are better. The above is just based on my experience.
Hopefully this helps - send me a U2U if you've got any other questions and I'll see if I can answer them.
Jiminy
[Edited on 17-1-2006 by Jiminy]