this problem happened all off a sudden, was fine and no problem to reverse one minuite and the next it engauges reverse but as the clutch is lifted its as if something is jamming it from goin back as the revs just die??? any ideas??

It's broke

Sounds like the main reason most don't bother with them.
Possibly spat its oil out and been runnng dry for a while?

I spoke to Quaife at Autosport this year about these as there is one in my car. There was a cutaway of one on display and the engineer from quaife explained the workings.

They are pretty bomb proof and the cogs that engage are very meaty.

Has it spewed all the oil out? (some do) - worth checking. Quaife advised it will work with minimal oil because of the over engineered gears if there is some oil left that won't necessarily have caused a fault.

As mine works fine can't offer any advice on the symptoms you describe - I would check all the linkages and phone quaife.