Thank you Gordon for sourcing AND sending on to me the required seals. It is very easy to get lazy with help like this.
Old seal removed and new one fitted within five minutes. Range testing revealed although the old seal appeared to be in good order using a jeweller's lupe, the new seal has made a difference and accuracy prevails again.
Next task, strip and service the loading bolt mechanism.
Walther Dominator; RWS LR20; HW100, HW97, HW97, Spartan