Hi Gareth,
I've worked at Distilled working almost exclusively on link building for a year, have spoken multiple times on link building, and have had a chance to run into the link building work of several people and agencies.
There are a lot of good link building companies and it may depend on your needs.
I can throw out a few names.
SEER Interactive - A nice sized agency run by Wil Reynolds. He's really well know for his great and creative link building presentations as various conferences. They have a team that includes a number of really solid link builders which should be more famous than they are. I'll say that when I went in-house, Wil's team is one of the first I pinged to talk strategy.
Citation Labs - Garrett French runs Citation Labs. I've never worked with him personally, but I've chatted with him. I've heard good things about his work.
Distilled - This is where I use to work. I'm a little bias and would say 3 months ago, it had great link building. 
My good friend Adria recently took a position there running the outreach. She's a great link builder and I recommend working with her. If you want to work with the UK office, I'd try to snag Paddy Moogan. If you're in the US, I'd try to snag Geoff, he's done some really solid infographic work. I made my first infographic with him.
Blue Glass - Bigger agency and I've heard good things about them from some close personal friends, but never seen their work.
Russ Hudgens - Good guy, really smart. I've never worked with him, but been on a panel with him, had drinks with him, and poked around some of his sites enough to see his work. He has some really creative and smart strategies.
Slingshot - I've work with them before with a client. They do some really interesting work and have a solid system for scalable link building. I've hung out with them at a few conferences, really fun team.
Outspoken Media - I've also worked with this team with a client. Rhea is amazing and I really like their entire team. They have a strong content based link building program and snag some really high quality links.
Kris Roadruck (Search Fanatics) - Previously at Click2Rank, Kris does some impressive scalable link building work. I've seen some of his work get solid results in highly competitive niches.
Virante - I haven't worked with them before, but I met some of their team in Boston. They sound like they do some really creative work, especially around linkbait type work. Some of the things I learned from them inspired some things I pushed for while at Distilled.
