Please post up the letters that you have had from TFL. Just redact your name & address, leave everything else visible.
Also any letters/email that you had from the London Tribunals.
Alas your car is a Euro 5 W204 diesel Mercedes. The NOx and Particulate Matter limits shown on the V5C exceed the Euro 6 limits, so it's non compliant for the ULEZ. It was worth checking as there are earlier cars that do meet the emissions standards even if they are registered as Euro 3 or Euro 5. And there are other reasons why the ULEZ database may not be accurate.
Petrol cars must be Euro 4 or have NOx emissions equal to, or less than, 0.08 grams per kilometre.
Diesel cars must be Euro 6 or have NOx emissions equal to, or less than, 0.08 grams per kilometre and Particulate Matter emissions equal to, or less than, 0.005 grams per kilometre.
These Low Emission Zones are being implemented all over the country. And also on the continent. Birmingham and Bristol catch people out every day. So check regularly
When sending correspondence to TFL or any other Local Authority then post 1st class at the Post Office counter, ie not in a postbox, and obtain a free of charge "Proof of Posting". Post before the last post collection and your letter will be deemed served (delivered) on the second working day after posting. Recorded and special delivery to a public body are usually a waste of money. Such post will be delivered in the same sack of post and may not even be signed for. Or use email where possible.
When you do submit the Statutory Declarations, one per PCN, the Traffic Enforcement Centre at Northampton County Court will will revoke the Orders for Recovery and order the Charge Certs cancelled. The case will be referred to the Adjudicator on how to proceed. The Adjudicator will likely ask you for copies of the representations you originally submitted. Probably the case will be held as an appeal.