Updating BRP help!!

Hi,

I'm updating my BRP following marriage. The mandatory documents to submit are my passport and proof of address.

On the list of "other documents" it states to send in an "original document showing grant of settlement, such as a vignette, passport or RON 60 letter". I have everything else apart from this (including all expired passports, current passport, marriage certificate, consent for background check and current BRP). I don't even know what a RON 60 letter is and can't find any answers online.

Can anyone clarify whether it's mandatory to submit this "original document ..." or is it just an extra piece of evidence to support my application? Surely my current BRP is proof that I already have ILR? This original document is the only thing I can't find in my pile of visa papers from the past few years.

Any help would be much appreciated!!