There will be lots that depends on the school / pressure on places etc. BUT, don't think that the maths is too low to get a place on appeal, others have managed it. You need the right evidence and ensure your appeal is tailored specifically to the school you have in mind (not relying too heavily on the potential 'twin' link). Somewhere there has been mention that there is a link between the NVR and Maths levels, which you would expect to be of similar scores but can't remember where I saw it.... Sorry
I got my DS a place at a GS with 2 failed papers, something many would think (and said) almost impossible but it was the evidence and circumstances surrounding this which won us the appeal. Some might say there was a place available so the panel elected to fill it but they concluded in their summing up that the school would be prejudiced by admitting DS although they felt our case was strong enough to over rule that aspect. We were denied a place at our first choice but DS was adament he wanted to go to the school he is now at, so we made a late application and the rest as they say is history.