Your 25p is generally ok - some will charge a bit more, some a bit less.
To test it out...... if now you are getting say 8 out of every 10 jobs that come along, up you price say to 30p. If your conversion rates fall then maybe you gone too far. If they stay the same then your ok and try a bit more.
However, even if your conversion rate falls, you may still be earning the same over less jobs - and theres nowt wrong with that.
Like Colin says, you be best perhaps with a minimum charge just in case someone says can you clean my downstairs loo carpet and you go in at 25 - 30 p per sq foot - err, £3.60 please - no, I dont think so.