the Rascal needs a 27.2mm post. dont use a 27.0, the seat post clamp is pretty fragile anyways. if the 27.2 does not fit it is very likely that someone has overtightened the seat post clamp and you may have to widen itvery carefully. is the slot at the back still parallel?
Just bought the frame off ebay, it came with a 27.0 post so assumed it was the right fit... Away at work at the mo but pretty sure everything was OK with the frame. Will be pictures of it on this website soonish...