Its all in the condition and looks. There is a HW collector's following which will pay good prices for rare and or good looking HW rifles.
If it looks great with some colour to the wood then it should be on par with a HW35E/Vixen so £350. Above that starts to get into competing with something truly special, so my gut feeling is £400 tops. No picture so my guess is £320.
What does a spanking good looking HW55 target go for?