Amazon have a WD Passport 250gig drive for a little over £81.
If you take it apart (very easy to do!), put the drive in your Macbook and then put your old macbook hdd into the passport caddy, you get a new disk for your macbook and an external disk for a very good price!
I did it with a 160gig disk and it took next to no time to do. You can pop the Passport's case open with a nectar card or tesco clubcard - it's just held together with plastic clips.
bohrsatom