UK Car Repair: vectra gearbox, hydraulic clutch, hot lunch

Question
I have a 96 Vectra 2ltr sri and at 30mph for no apparent reason there was a loud bang from the gearbox and there is now a hole in the back of it and oil everywhere. I have to try and source a second hand unit but I cannot read the identification number on the top of the gearbox.
I know that the car has a hydraulic clutch but I need to know which gearbox should be fitted to this model. The breakers have told me that it could be f17,f18, or f20 - any ideas which one is correct ?

Answer
The reason it went bang and KO'ed a hole in the side is that it was fed a hot lunch or two in its lifetime and the newer automobiles cannot handle it. This car is no American car. The simple answer to the question is... taking it out and bringing it along for comparison. You have to take it out anyway, right? And you would like them to have your old unit so you don't have it kicking around, right? So, take it out ad tell them you’re going to bring the old one with you for comparison. It is a small unit as far as transmissions go and should only weighs around 125 LBS or so, two people if not one should be able to move it around easily. Remember to take replace the clutch with a new one while your in there. And if you do not have a book for this car already you will need one.

Regards,

John