This is an impressive machine, it’s at the Museum of English Rural Life, part of the University of Reading.
Described as being designed and made in 1893 by James Sumner of Leyland, Lancashire who subsequently formed the Lancashire Steam Motor Company with Henry Spurrier. The engine and boiler are fitted on the machine so that the bulk of the weight is on the main roller, and the machine being nearly balanced is easy to turn. This particular lawnmower was used on Lord Harrington's estate, Elvaston Castle, Derby from c.1894-1929
This is the only mower in this static exhibit indoor museum but well worth a visit for a couple of hours if you are in the area. The museum has a very large collections of horse drawn wagons and ploughs. https://merl.reading.ac.uk/