I have to agree, I've just done two specific searches and got nowhere.
It used to be quite good, but the search facility on this forum at present seems to throw up everything apart from what you are looking for.
Try Google, using either 'Webley Mk.1 Air Rifle' or 'Webley Service Mk.11', or variations of those words, and you might have more luck, as it often links to older threads/posts on this site.