In reply to Dave 88:
I've just done this. My storage place charges £1 per week per £1000 in storage which will be £4/week for you. It's part of the business model.
There is one online place I found that charges 55p/£1000 which seems to be the only one available anywhere and a bit more reasonable. Can't remember the name but google it.
Some storage places insist on you using theirs. Mine don't.