Thanks for all the replies... I should have known that if I asked a group of pilots I'd get lots of different answers!
Building my own PC isn't an option. It would take me too long (I don't have much spare time anyway), I wouldn't be confident about it and, if something went wrong, I'd have no-one to turn to for support. I have thought about it, and decided not to.
I get the impression that Dell are good for business users, but their customer support is somewhat lacking for personal buyers. However, Mesh seems to have a good reputation. How much 'fiddling' with spec' can you expect a company like this to do? i.e. if I wanted a model as advertised, but with a different screen and more memory, would they do that?
Thank you again!