Hi Jago
I'm in a similar position to you, the 1st clean tends to take ages and the daily rate can be as low as £90. But, if you do a thorough job on the 1st clean, by the 3rd visit you should seem a distinct improvement in your hourly rate.
Even in areas that I consider 'slow' my hourly rate is £18-£20 per hour and I usually make between £140 - £160 per day.
I tried canvassing bigger houses and charging a higher rate, but to be honest there doesn't seem to be any clear benefit in the hourly returns, in fact I think it's better to do 3 small houses at £6, rather than a couple of medium-sized ones at £9.
Having said that, my hourly rate for the last few days has been a big, fat, hairy 0, as my car has bitten the dust and is receiving such much needed TLC. Steer clear of French engineering is my advice
Rik