Attest

Yes I have had to obtain something similar that declares I am not currently married, which is a standard document in Belgium nothing like it exists in the UK.

I went to the UK Embassy in Brussels (Dont believe the website or security gaurds you do NOT need an appointment, just be aware of the strange opening hours)
I litterally had to write out on a piece of paper my name and that I was not married. It was then typed up, stamped, and I signed it.

Bear in mind that it costs 50 Euros....

Thanks for replying Vince. Eventually I came to the same conclusion and phoned the consulate. My attest is actually needed to prove that im the same person on the new passport that held the first passport - the guy there laughed and said it was silly because the passport is the attest itself because they have received and checked all documents and issued a new passport in that name. Even if I choose the option to record my previous name on the new passport the Belgian authority will not accept it. So, he suggested the same as you did, come to them and they will do it but it will cost me 72 euro. I have to first check what info they want on it ofc and the belgians are just so evasive. It's like they really enjoy this game. *sigh*