Marston are the new name of 'Drakes' they are the bailiffs that got all the bad press after a TV documentary not so long ago, they bought up a small company and now use their name to clean up their image. Firstly, NEVER ever let a Bailiff into your home. Secondly, it appears the court probably did write to you at your previous address, because you never responded the debt was passed to Marston's / Drakes, they've traced you to your new address and now seek payment. Last but not least I suggest you write to the Post Office (I believe their investigation section is at Chesterfield) tell them which Post Office you purchased the Postal Order at, Date , Amount and whom payable to, I'm sure they can trace it. Surprisingly you are probably the only one to buy one for that amount on that date so then they'll have the Serial Number. The £20 Marston's are after is for the balance of the £120 fine imposed by the magistrates court, not you defaulting on the CEE scheme.