Thanks bazza, sounds like there might be some room for manouevre here, which is all promising. I'm well used to the odd looks when taking out the (already thin) seat cushion so I sit on the metal frame at track days, being able to take the cushion out sounds good to me.
Once there is a frame/seat available, I'll make the trip for a fitting and see what's what. (Thanks for the offer Ary).
I suppose I should ask - as I'm the only person likely to drive my car, would I get any more room if bolted straight in rather than with rails, are there any options here? (For reference, I'm 6'6, long in the body and have more of a relationship with my headlining than I'd like..)