Hi - do make sure they are fully insured and trading legally as self-employed (ie tax registered) otherwise the liability issues are potentially huge.
Word of mouth is the best source of referral. I wouldn't risk using someone who put an ad up in a shop unless they could show you insurance documents.
Never pay cash in hand as paying into the bank will expose and flush out those trading illegally.
I would expect to pay between £12 and £15 pound per hour for a legal operation but occasionally some will do it for less for a time. Anyone really good will have a waiting list unless you can catch them as a start-up before they get booked up. It's very much worth joining a waiting list rather than hiring the wrong service.
Make sure they let you know what is included within their service and what is additional when they come round to quote your home. Also they should let you know what is beyond the scope of their services and insurance cover.
Occasionally a cleaning service will also offer laundry tasks and even the odd one does gardening as a bolt-on service, and with the occasional service, carpet cleaning can be arranged by that same service too, same with oven cleaning, this can sometimes be arranged as an add-on by the same provider or some will outsource these, all are different so make sure it's all clear from the start.
Most are dedicated domestic cleaning services especially if the operate solo or in a team of just two, they will not be able to spread themselves too thin with what they can offer and will specialise in this field only and not offer the additionals for which they do not have the man-power.
Once you know what that particular service offers you can see if it fits your own requirements and at some point in your search you will find a good match for one another. It's no use hiring a service who does not offer ovens if that is what you need regularly, you need to make sure what they offer is everything you want.
The vast majority of clients just want the regular domestic cleaning service but if you want any extras make sure they are able to offer these.