There will be many factors to consider, to name a few: age of children, location & availabilty of babysitters, your confidence level etc.
With young children, I expect you will want a babysitter who knows about children, ideally someone who works with children on a regular basis. Someone who is trainned in childcare, first aid, and who has some maturity so they are more like a parent subsitute, rather than an older sibling.
With older children (say aged 7+), you may feel the children would prefer someone who was more like an older sister/brother, so being trained isn't to vital. However you would ideally still want someone who will cope in a crisis, who can deal with childrens minor upsets. Teenagers with a more mature attitude can be good for this, as long as you have the confidence to leave your children in their care.
I'm on the Surrey/Berkshire border and have been babysitting for over 16 years! Getting old now These days it costs parents typically £30-£50 per evening (3-5 hours). This is quite a high price to pay, but they do get a fully trained, experienced children's nanny, who provides their own transport and doesn't mind staying till the crack of dawn!