Peter Johnson has sent me some further notes and they are certainly worth clipping in for a wider audience:
The ZSPC series is documents transferred from the old BR records office so Hayward's collection was obviously transferred to the BTC. I understand that when the BTC collection was dispersed, Kew had the company documents and the NRM had anything dealing with locomotives and rolling stock. The PRO/NA is not a repository for photographs for their own sake so would not want them. My guess is that the bulk of the collection is at Kew. If anyone took the trouble to count every item in ZSPC11, every document, ticket, cutting and photograph, the total would approach the 18,000 mentioned. I photographed over 300 items in the Shropshire & Montgomeryshire Light Railway collection alone.
What Peter says about the splitting of the BTC collection makes sense and explains why Tim Procter the NRM archivist has come across lantern slides marked Hayward. Tim, by the way, tells me that having had the chance to read this and having found these lantern slides, some timetables he had found which he thought were Hayward (and which an earlier entry here refers to) are probably not.
A further update from Peter Johnson which I am adding on the 28th April 2010:
A little more on the Weh-lyn collection that I thought would interest you.
From the Journal of the Railway & Canal Historical Society March 1974:
New accessions to record offices, 1968-1971
British Transport Historical Records
Private collection: W. E. Hayward: collection including 776 files of articles, press files, railway ticket albums, Railway Clearing House maps and junction diagrams; 250 books relating to railways".