Agencies vary. You pay them to find you candidates, ideally a couple, the best of the best, so you don't waste time interviewing unsuitable people. They should carry out the main reference checking and eligibility to work, qualifications checks. If you specify you want a driver, then they should check the nanny has a driving license but may not go in to exactly what vehicles they can drive. What new driving license check?
Contracts may not be their specialist thing, that would be more from a payroll company who employs a specialist lawyer who can draw you up a custom contract. NannyPaye do that, think some of the other payroll companies might or they may only provide a basic template. Check with your chosen payroll provider as to what services they offer and how must customisation of a contract is possible - it needs to be legally enforceable so will be comprised out of various legal phases.
£450 sounds quite cheap, some charge thousands! Their terms of business should say what services they provide for the money. The matching service is the main thing and doing the background checks. You should be getting a few suitable candidates, not having to interview lots, so by the sound of it they have done that part alright.
They should also offer general advice about being an employer of a nanny, what you need to do to have a good working relationship, answer questions about expected duties, typical salaries in the area.
What were you expecting them to do, have they met your expectations or only partially met them? Could you negotiate the fee if they did not meet something you expected and which they claimed to offer?