Lee,
I'm going to pre-empt the mods here and refer you again to the following thread:
http://www.bushcraftuk.com/forum/showthread.php?t=53186
Sam gave you some links to possible solutions but he qualified it by saying that he used this method on "estate quads" - not on public roads.
It's an unfortunate fact that the public at large are alarmed by the presence of knives and guns and we owe it to the survival of our pastime to be courteous by not outwardly displaying the tools of our trade, as it were.
On to biking... I am an experienced (and still alive) biker - I've road raced, I've track raced and I've (very) long distanced toured and one thing I've learned is that Sod's Law clearly dictates that Mr Plod will pull you over when you least want him to. It's a fact of life that most bikers get pulled over in the summer (hunting season?) because thats when a lot of us get killed on public roads. Thankfully, nowadays, if ever I get pulled over - it's normally by a motorcycle policeman who just wants to talk "bikes", I get some top tips on road riding (they are the experts after all) and I get to go on my way.
If you get pulled with a filled gun case modification on your bike - I can pretty much guarantee that the conversation is not going to go your way! (you need to explore the MOT and insurance laws on this as well).
Believe me mate, I have explored this to the n'th degree and it's a non starter for public roads - I'll say again unless you can conceal your rifle in a rucksack sized pack (and I would suggest that modifying an S200 is going to reduce the accuracy of a very fine rifle) then don't go there - biking and air rifling do not mix unless you have a TDR (I can't bear to part with my Theoben .20 MFR - so car it is!)
Regards
Fin