Neil, thank you for the photos of the lock interior.
No doubt the actual model of the lock will be revealed by the experts in that field.
I think Chubby and magpie are as close to the true facts as we’re likely to get. If it was made by Tann they should be ashamed of themselves. Judging by the apparent depth of the door rebate a stronger wedge-resistant 1″ thick made-up door appears to be fitted instead of the weaker ½″ single iron plate, yet the bolts fixing the lockcase are pathetically inadequate to prevent dismemberment.
Then to put the tin lid on it a top class Chubb 9-lever (?) lock is fitted. So, as suggested, this could have been a retro-fit which was not an infrequent practice. The Tann account books often mention the fitting of such as Gibbon. Barron and Chubb locks at the customer’s request with the latter being priced at £10.0.0. In purchasing power terms that’s around £870 today. Not exactly balanced security.