Encircled N On British Cover

Hello All,
We have been asked about a postal marking that consists of a letter N within a circle as illustrated on the attached cover.

It is believed by the owner that it is some reference to the item being redirected.

We have a fair number of reference books including the 9th Edition of S.G. Collect British Postmarks but can find no reference to this mark anywhere.

If anyone can offer any advice we would be most grateful.
Thanks.
Ron and Eunice

Hello All,
For anyone interested in this thread we have received a reply regarding the marking so thought we would post it here.

Quote
Thanks for yours. The mark in question is an example of an Inspectors mark which come in many forms and were applied to mails from time to time. They are of interest to some specialist collectors of postal history but generally don't add value to the items upon which they are found.

Unquote
Peter Mollett has a cover with an encircled K in his latest list (No 154)
So thanks to Peter and to Robin Restall who passed on the response.

Regards
Ron