Thanks for all the replies.
To Paul L and Dalboy for links to the UNO lookalike as Circlip notes. Like Circlip I remember the UNO from Drawing Office days and have a fair selection of them in the back of the drawer still. I have tried them on paint and the results were not great, probably need more clearance to function on paint. To be fair, they weren't great on ink either – the Rotring was like waking from a nightmare!
Anyway, the real problem is that I need to do 1/4" lines and while twin lines with a UNO and fill in the gap with a brush is possible it is very slow. Some of my 4" Burrell was done that way with a draughtsman's pen rather than a UNO.
Thanks to KWIL and Paul R for the experience on lining tapes. I might go that route as I do have some curves to do.
I went down the scrap mine last night to see if a Dave Fenner pen might appear and I have found some scrapium that might be suitable.
However, there is a more interesting possibility as a result of this thread, which I will tell you about if it comes to fruition.
Thanks to everybody, an excellent forum as ever,
Martin