Is it anything like THIS? This is a Beeman Sport with Blue Ribbon scope (with Apel QD mounts).
I have been offered £400 for this rifle and refused.
The 'Sport is becoming a much sought after rifle again in this country and I have heard of nice examples going for £200+. Your Beeman additions, the .177 cal and the Blue Ribbon scope should, IMO (depending on condition) make it worth in the region of £300.
HTH
Ian