Namely, a Bell Tent Co inner to a Soulpad bell tent? There's one I'm stalking on ebay. 
I know they hang from the loop at the top of the central pole, but Bell Tent Co say (well they would, wouldn't they?) that their inners will only be compatible with the 'bespoke fittings' of their own tents. 
Are there any other fixing points for inners, or is it just the loop at the top? Presume they also toggle onto the loops at the tops of the walls - I wonder if this is where the difference comes in?