Saw this on eBay, may be of use to someone on here:

Vauxhall Omega Estate Low Mileage Engine 2.5L V6 manual on eBay (end time 19-Feb-11 09:48:02 GMT)

Seems to be a pretty honest description of a crap but currently legally driveable car with a decent engine.

I suspect the gearbox is a GM item?

the older senators had good gearboxes apparently.

the 3.0 boxes are sought after for performance cars.

Opel's own 4-speed manual transmission was not up to the job and, instead of putting in a more modern 5-speed manual gearbox, Opel turned to transmission producer Getrag, and installed their 264 4-speed manual gearbox in the early Monzas. This was soon replaced with the Getrag 240 for the 2.5 and 2.8 engines, and the Getrag 265 for the 3.0E; both 5-speed manual gearboxes.