Answered in 'blame the manual'. Thanks.
"Create Meshes: A7 Compiles the level in A7 mode, with static meshes rather than polygon based blocks. Required for the A7 light manager, or when shaders are applied to level surfaces. This box should normally always be checked. Level rendering can be greatly sped up by combining block groups to single meshes."
Guess that's the price of being used to an old version only
