It looks like all his companies in Finchley are at the same address.
Don't bother trying to find "St James's Lettings" at http://www.companieshouse.gov.uk/ - it doesn't exist. Guess what? Company called "Marcus Holdings Limited" (06856276) still exists so all that is a good company if you had problems with Martin Marcus more than 1 year ago. If you had problems with Martin Marcus recently then he might trade as "City Lets (Finchley) Limited" (07447216) - check http://www.companieshouse.gov.uk/ or download certificate of incorporation for free at https://docs.google.com/viewer?a=v&pid=explorer&chrome=true&srcid=0B3gY1hY8GkvyYzllZGYyZGEtZmFkMC00NDllLThlZjktMDIyNzAyZWYyYjlj&hl=en_GB
Feel free to use Money Claim Online at https://www.moneyclaim.gov.uk/web/mcol/welcome
Many people (more than 100 now) are contacting me about problems they have with getting money back from Martin Marcus. I strongly advise you to ready my blog to avoid problems. If it's too late then don't hesitate to contact me at citylivinghackney@googlemail.com and report your problem to the Police and Trading Standards.