Catnip, Sitters to my knowledge does not normally do daytime care though I think they can arrange it occasionally, so check with the agency if they are able to provide care for the times you require.
Plus Points:
# You are very likely to get someone to babysit - national agencies have lots of people on their books. However, it does depend on location, more availability in Cities/Towns v rural villages.
# You don't need to check the persons background yourself as the agency will have already done this. Though I would suggest you still at least ask to see qualification certificate(s), First Aid training certificate, latest Enhanced CRB check (ideally not more than 12 months old).
# The agency should match the babysitter's experience to the age of your child. So as you have a baby, request someone with lots of baby experience.
Negative Points:
# You have an on-going membership fee to pay, so if you do not plan to use the service after the wedding, make sure you cancel your membership.
# The national agencies tend to only provide Evening Babysitters, typically working from say 7pm to 11pm. If you are wanting hours different to this, then check the terms & conditions carefully, as some agencies may have different rates for after midnight, or for childcare before 7pm. Sitters charge extra for sittings starting before 6pm, and for sitting at locations other than your own home (hotels, temporary addresses)
# It may be tricky to arrange to meet your babysitter prior to the actual evening, due to the babysitters daytime job. Therefore the babysitter will be unknown to your DS, which I personally feel is not good for babies. When I babysit for families for the first time, I always meet the parents and children first, so that when I babysit, I'm not a complete stranger - this does however add quite a delay to the booking process, but I feel it's for everyone's peace of mind - the parents, the children and my own.
# Cost wise, the hourly rate charged by agencies can appear quite low. However once you add in the Membership Fee and Booking Fee, plus any other additional fees, the actual cost can be quite high - especially if you use the service infrequently. You tend to get best value from agency services when you use them on a frequent basis. I babysit in the Bracknell / Wokingham / Camberley / Guildford area and I found that my fee compared very well with Sitters, if parents needed a babysitter once a month.