Katherine,
Kev knows I'm from Pembrokeshire, and have close links to Pembrokeshire Bird Group and some of their members.
This is a good link:
http://www.welshwildlife.org/attachments/Skomer%20Bird%20Leaflets/Skomer%20Timeline%a9Peter%20Gill%20%26%20Associates.pdfIts a timeline for Skomer wildlife and your timing will be spot on. Puffins will come up to your feet and wait for you to move so they can access their nests!
I have links to Pembs Bird Group and older posts on my blog have Skomer information. Boat trips around the islands are good too, Kev mentioned late ones as sometimes the Puffin / Manx Shearwater rafts are an awesome sight.
Hope this helps a little.