We've used an agency called EMA for the last 3 years. We've had a few difference cleaners from them over the years - some better than others - but all reliable and with references.
I think this is their website - http://www.emadomhelp.co.uk/ - but it looks a bit out of date (price is now £8.50 p/h unfortunately).
I've messaged you with the contact details I have - although I think the one on the website would work as well.