I enquired of the editor of Railway Modeller and received the following response
Dear Rik,
many thanks for your message. Steve's on holiday at present, and I'm looking after the inbox for the time being. The answer is there were two articles! The April fool ? one of the best we ever did, I think ? was in the April 1983 issue, but the other underground feature was by a Dutchman, C R Tiddens, whose Piccadilly Line models did in fact run under his existing layout. They were featured in RM October 1978.
Kind regards,
Tim Rayner
pp.Steve Flint
It seems there were two articles.
Rik