My document isn't to hand (it's in the cupboard under the stairs and the dog is on his bean-bag in front of it), but you'll get 1 million pounds worth of public liability for around £100.
Some largish commercial work may require you to have more; five million is the usual amount, but this'll cost more obviously.
One million should be enough for a mainly residential round.
This insurance doesn't protect you if you break a window, but should your ladders blow off a wall and land on someones head/car/caravan; it'll come in handy.
It's tax-deductable too.
Also, you can purchase insurance to cover the cost of breaking a window; but again; it'll cost you more; obviously.