Rear der. conundram

I fitted a Shimano Megarange cassette to my road bike (Dawes Comp.
Giro 300) to try and ease the symptoms from my knees. In order to take
up the larger chain slack I also changed the Sora 8 der. for an Alivio
8 der.

I can't get the der. to change to the low sprocket, apparently because
the jockey wheel isn't far enough out from the axle. The 'b' screw is
fully in. What options do I have? TIA.