its how long is a piece of string ?
how fast do you work? how do you price, what size houses etc
you may have low overheads and be able to live quite comfortably on that, also what demographic are your customers, what competition do you have?
but i would reckon you could probably get too £25 an hour quite easily, ie price rise, some new better priced work, and refine your techniques a bit are all possible in order to boost earnings.
i take you are unsatisfied with £20 ph ??