For evenings you need a reliable, responsible, and as your baby is young, ideally you want someone with childcare training and experience of caring for babies. So that makes me ideal - though I don't travel as far as Reading... don't suppose you are nearer Bracknell area are you?
Alas I don't know of anyone like me in the Reading area, so can't recommend anyone to you. Another regular poster on here nearer to Reading may know though, so keep an eye out should she decide to post a message.
There are national agencies around who could help you, though you will want to make it clear to them how old your baby is, so that you get someone with the appropriate experience. Not all childcarers will have cared for a baby under 3 months old. In the Reading area you may also find there are nannies who work with young children during the day, and whom could do a few hours for the odd evening.
Some useful places:
www.nannyjob.co.uk has a babysitter search facility, most of whom I would expect are nannies.
www.findasitter.co.uk lists approved childcarers.
reading.gumtree.com may be worth a try, as you can sometimes find adverts there from nannies... plus you could try advertising there yourself.
Mumsnet Nanny Shares board can also be useful, as it lists many different types of childcare provider.
Hope that's of some help.