Looking at a couple of images of them Id say theres not much problem with scope length Simon
the ports between the front screw and the chequering on the image
http://www.countrywaygunshop.co.uk/product/weihrauch-hw57-air-rifle/
would probs be the same as a 97/77 need to leave room to get the chubby digits in
Are you employed by said pest company or sub contract?
if employed then surely its their duty to get you the required certs to operate on their contracts
if your self employed then only you can way up the cost against gains for getting the certs required.
Nothing stopping your company running a in house course is there?