Doing a bit of research online, i came across this photo :
For the purposes of crediting the original photographer and avoiding any copyright problems, here's the thread it was on :
http://forums.turbobricks.com/showthread.php?t=319179
It answers my question anyway, there's obviously a plug i can't see because of the position of it.
As soon as i can shift the chewed nut on the prop/diff flange i can get on with the rest of the axle swap. As long as i don't encounter problems with the handbrake it should be a simple spanner job. Everything else has been loosened (and tightened back up again!) so i just need to find a replacement M10 Nyloc for the chewed one and a bucketful each of energy and enthusiasm to do the job!
Thanks for your help and input Kay!