While i have the gerabox out i decided it would be best to take it apart and check it over and replace all the bits i could like seals, speedo drive, reversing switch etc..
I got most of the parts from eBay Germany from Thomas. he has an amazing Ascona and sells lots of good qualityy replacement parts for Opel Mantas. Check out his website here and his eBay shop. He is very helpful with any questions you have and the parts arrive very quickly. I also found a very handy parts view of the gearbox just in case you decide to take yours apart! Here it is and there is a link to the original source info.
The box is now back together. If your thinking of taking one apart (the easy bit) make sure your not in a rush when you put it all back together, its not a quick thing and i found putting the bell housing on the floor and lowering the back part of the box worked as i just couldnt get it to line up doing it the other way