Thank you all for the comments, very much appreciated.
Mixture, one reason I wasn't keen on the idea of using Teamviewer or similar was that I don't normally leave my PC running when away from home; the remote access would go directly from the internet (via my router) to the DVR. My concern of course, particularly with my limited knowledge, was that when I did have the PC running that somebody could enter it via the new router port. Incidentally, with my ignorance of the significance of settings, the present router port number I chose was 81; probably not very clever!
Thanks .... Jim