Bevel won't go above certain size Sorry if I've overlooked something simple but this is driving me crazy. I'm just trying to apply a bevel modifier - but there's a limit to how large I can make it.

Please see the image to see what I mean, paying particular attention to the width value of the modifier.
Disable the clamp overlap option. Clamp overlap prevents the bevelled edges from overlapping each other (typically an indication that you don't want to exceed that bevel width).

If you're dealing with bad artifacts after exceeding the clamp overlap limit, this is most likely due to bevelling of a very acute angle. Try using the weight or vertex group limit methods to only bevel other edges in that case.