Andrew,
Icesave offers UK savers the same level of protection as any UK registered bank and building society. Icesave are what's known as a European Economic Area ("EEA") Bank, and they have topped up into the UK's financial services compensation scheme, which means you'll get the same level of protection as you would do if you had your savings with a UK bank, up to the current level of £35,000.
For EEA banks like Icesave the financial compensation scheme is provided by two separate schemes e.g.
The Icelandic Depositors' and Investors' Guarantee fund provides the first level of protection up to 100% of the first €20,887 (or Sterling equivalent); and the UK Financial Services scheme tops up your protection so that under both schemes your protection is equal to 100% of the first £35,000.