Not sure of precise dates but they were definitely available in the Uk from the '70s to around 1990. After the wall came down, supplies of Haenels started drying up. Parker Hale were the importer for a time.
The decent 303 should be putting out 7 ft/lbs. Perhaps a shade more. Haenels are fine, well made guns IMO. I've always had a soft spot for them. Spares are available from Chambers.