If any one can help fix this it would be great

I can separate all the surfaces and join them back up again, starting with inside surfaces and working my way round. Once its completed it says it's solid, but look down the middle and i have a strange surface generated from nowhere, so i delete that surface and replace it with a blend, now it wont join again, explode to surfaces and repeat, i am going nuts trying to fix this.

The surface that has issues is magenta.

Martin Spencer-Ford