They are very rare and worth quite alot, £400 to £500. They are collector's items and as they are high-quality pieces, they attract high prices. I think they are based on the rimfire ERMA used to make, I shot one of those in a gallery once, nice solid rifle.
https://forum.vintageairgunsgallery....-10-air-rifle/
Have a look at this old thread discussing the air rifle ...
http://www.airgunbbs.com/showthread....highlight=erma