It might be worth looking at your home contents insurance to see if they can offer a deal. If you're not using it for business then usually you can add it as an extra item.
If not one company I've heard of is
LaptopGuard. Their website looks a little broke on firefox though, but don't let that put you off
Prices
Policy details:
Laptopguard insurance is suitable for:
Anyone over the age of 16 who wants to insure their personal laptop.
Any laptop less than 5 years old.
Laptopguard provides:
* Cover against theft and accidental damage up to £3,000 (items over 18 months old are subject to a deduction for depreciation)
* Cover for specified accessories
* Replacement on a new for old basis (on items up to 18 months old)
* No excess as standard
* In vehicle cover to protect your laptop whilst you travel
* Area cover with your choice of:
o UK only
o EU (including 30 days worldwide cover)
o Worldwide