Eureka! I think I've finally nailed it down. As well as John's photo, I tracked down another photo of the pottery:
https://www.ebay.co.uk/itm/274675188286. This gave me the railway line and junction, and I've been up and down the line through Queenborough but I couldn't find the junction with a works behind. Then by chance I hit on
http://queenboroughsociety.org.uk/ and right at the bottom of the page is a map "Early Rushenden 1934" and it shows the pottery works. The works hadn't been built when the 25" 1892-1914 map had been made, which is why I couldn't find them. They are on the 1:25,000 1937-61 (6") map, but are unlabelled. However the 1:1,250/1:2,500 1944-1971 maps (50" and 25") do show them labelled! The railway line (and I should have spotted the single track) is labelled "Mineral Railway" with the junction south-west of the works. Looking at the roofline the steep slopes are to the north, so John's photo must have been taken from the east. The building (which John identified as the yard foreman's house) must have been one of the small buildings immediately in front of the works, alongside Rushenden Road. Tentatively the location is:
- Nat Grid: TQ 90993 71670
- Nearest Post Code: ME11 5AU
- Co-ords (WGS84): 51°24'43"N, 000°44'42"E or 51.41188,0.74502
