I've used Auto Trader adverts in the past to get a sensible valuation on a stolen van where insurance loss adjuster just wanted to use guide book prices. Van was quite rare and "for sale" prices reflected the uplift over guide books
If it's for insurance purposes then some companies have an in-house valuation service (for a fee) which usually requires you to provide six specified photos. An idea of what you think it's worth is never a bad idea and as long as it's not crazy they will likely agree it.
If it's needed to be done by an authorised third party then membership of the Mercedes Owners Club gives a list of member valuers who issue a certificate for a £24 fee to the Club. There is one local to you in Essex who is particularly expert.