Fit the biggest you can.

Even though what you've got might only draw 22amps, it's always better to allow for future purchases as best you can. If you only fit the bare minimum, the chances are it'll cost you a lot more in the long term.

In the latest TLC catalogue, there's only £27 difference between 50 metres of 4 and 6mm2 two core armoured, and £30 for three core.