Not in Sheffield so don't know costs there but I think your first thing is to determine if this is babysitting (occasional evening care typically between 6pm and 2am), occasional nanny (daytime care at your home) or something more permanent (part-time nanny caring for your child at your home).
How often it occurs and the amount of commitment required affects the employment status, which would effect your cost.
3 hours on 3 times a week on the same days most weeks would be more likely employment as a part-time nanny than as an ad-hoc babysitter.
If you want someone to come for 3 hours one week, then none the following week, then 6 hours the next week, then none for several weeks, then 9 hours the next week - so it constantly varies and the person doing it can say "no, can't do Tuesday next week but I could do Thursday is that of any use?" then it may be them offering you a service.
So I would determine that first as it would effect your legal responsibilities and thus your cost.
Then establish what costs other people in your area pay for that type of care. Then if you are providing a job you tell the childcarer how much you are paying, or if they are offering you a service then they tell you how much they charge for their service.