sil0 already posted this (http://www.hotukdeals.com/deals/seagate-500gb-expansion-portable-ha/744905) however I thought it deserved a little more attention considering we can get £10 off, as follows:
Simply add a cheapy mouse mat for 58p (http://direct.tesco.com/q/R.207-0389.aspx) to bring the order over £50. Then use voucher code TDX-RTHK at the checkout to get this pretty nice price!
Edit: They are out of mousemats but just look for something cheap like "value dvd-r 5 pack" (£1) or "Tesco Value Modem Cable" (£0.38) to get it over £50..
This is a nice alternative if you want a USB powered drive, however if you don't mind it being powered and chunkier then I'd grab this one posted by robappleby89 instead:http://www.hotukdeals.com/deals/western-digital-1tb-hard-drive-44-9/757853