Thanks both for your replies!
I'd seen the Rushden Hearts and Soles website but not the one for the museum.
I've now popped an email off to the museum to see if they have any ideas, and I guess I could also email the Rushden & District Society. For the latter, I think I'll wait till I hopefully hear from the Park Road Baptist Church as to whether they happen to have the books or not (Ringstead Baptist Church does still retain its own books interestingly).
For information, I've also tried the following catalogues to no avail:
TNA Discovery
Angus Library and Archives
Evangelical Church (including the Strict Baptist Historical Library)
Northants Archives.
Fingers crossed ...