Many thanks for the warm welcome grandarog, and for your helpful comments MartinR and Beachbum.
This appears to be a Warrant, which gave powers to operate. I have a simular Warrant from my Service.
I'll ask my son to scan the Warrant for me today on his machine and forward me a copy as he should know how to get the best resolution and I'll then upload asap. So glad to now have a descriptor for the document and confirmation of which Customs House issued it, as hopefully this might assist in narrowing down my search for any further information local archives may hold. I've tried to research the other signatories, and according to the Freemen/Poll books of the time, Mr Booth was a Chandler based in Maidstone, located a few doors down from the Maidstone Customs House, and Mr Richmond was also a Maidstone trader so seems interesting that they then are witnesses to a document issued at Rochester Customs House.
Regarding the Security mentioned, on this basis, would the Customs House in Rochester not have expected the Warrant to have been returned at the end of my ancestors employ, or was it commonplace for these to be kept? Since I could imagine that they risk being used at the time for less than honourable purposes should the former employee fall on hard times for example. During my visit to the National Archives, I was able to request the superannunation records for Customs Rochester for the relevant time period and several boatmen were mentioned but alas Mr Clegg was not one of them so seemingly no pension and no request made by his widow.
In case of any interest have attached to this message the article I found that relates to Mr Cleggs widow Elizabeth, who seems to have died rather tragically. I have made a search online of the coroners reports held by the Kent Archive centre, but sadly hers in not one of the reports that they hold. Again given that she died in poverty, it makes the survial of this document all the more strange/remarkable to me. My direct ancestry to David Clegg is via his second born son William (a bargebuilder), and the Cleggs appear to have been mostly known for running the Gingerbread Shop in Maidstone up until Davids father Daniel who became a Waterman in the mid-1700's and most male decendents then followed suit re: boatmen or bargebuilding etc. All seem to have stayed local, and the family plot is at All Saints church, but David does not appear to have been buried there and I assume Elizabeth would sadly have had a paupers grave based on the article attached.