My bad, however adjustment is still the answer.He's talking about the front - not the back.
With the new crankset the small cog is only about 2mm away from the back forks and the mech doesn't drop down that far. I need a spacer between the frame and the crankset
If I get a bb with a longer axle will that not just be longer on the outside? My mech won't go in far enough so I was thinking that the crankset needs to come out further?That looks like an internal/square taper BB, which means that fitting a spacer is not an option. You need a BB with a longer axle. Whatever chainset you have should also tell you somewhere what length BB you need.
The old one came with the bike from new. As did the bb,Your old crankset probably wasn't a triple, or was designed to fit with a different length BB.