Define dodgy, or would that be libellous?
I am
assuming you mean Chelsea Cars....My experience is that they sell a wide range of cars, not all of which are worth the money - as in you can find better examples elsewhere for less cash. They have a name, a position and the like. I have heard similar things about Classic Chrome, too.
If you're looking for a 113, I would suggest:
a)
www.silverarrows.co.uk - Michael is expensive, but generally has some good cars and knows what he's talking about. He's also round the corner in Putney
b) join the MB Club
http://www.mercedes-benz-club.co.uk/ For the £30 odd quid a year it costs you, you'll get a cracking magazine monthly, with a fair few Pagodas privately sold every month
c)
www.mercseller.com - An odd "putting buyers and sellers together" site/service this but they do get some decent cars go through their books. BUT....
before you do any of that, have a good read and site search on here so you have a vague clue as to what to look for. Do that and you may well find a complete gem in Wandsworth.