It is a difficult one as it is so variable as you have experienced in fact. On a short job say yesterday fault finding on a ring I charged £35 per hour sorted it in three hours out the door with the cash. On jobs I do for a regular supplier of work £200 per day. On commercial £300 per day. When someone looks like they have a problem paying a tenner will do for an hours work. Just depends on the client, length of work, domestic or commercial, one off or regular, short term or long term, complexity and simplicity and so on. I feel my way with each client. Bottom line it is whatever the market will bear. £21 per hour is not bad for northerly climes from what I hear. £25 per hour would be better. I think it is all down to what will get you out of bed each day, if is not enough, let the next man take the work. From what I see there is loads of work out there/here.