Marine Electronics Services, they do recommend that you install yourself but provide lots of advice and will install if you want it. www.mesltd.co.uk We found them to be the cheapest quote for Icom 710 + accessories.
Will second that advice for almost any electronics, because they deal with a very wide range they don't push one make and advice always seams genuine and solid.
There was a previous post on SSB instalations and which unit which you should search there are some very relevant comments which should be taken on board.