The Howard Gem rotavator as seen on the The Good Life was a series V machine with a Kohler K341T petrol engine.I would guess that particular Gem was a very late 1960,s / early 1970,s model as it looked a bit beat up even when The Good Life was first aired in 1975.
Definitely worth keeping and using unless its totally worn out in which case it will cost £££,s to put right !.Spare parts availability is very good so you should have few problems in that area.Now lets see some pics :):).