Auto box-reverse gear

Evening all. My D2 v8 has been back from the fixers a month now. While there, it needed a new transmission oil cooler as the old one split (its done 110,000 miles and is a 2000MY), and the oil was changed in the auto gearbox while they were there.
I thought it was imagination that the reverse gear was fading but today, I have lost reverse all together. It changes gear on the dash display, you can feel the prop shafts reverse but there is no drive to make it go backwards.
Any ideas before I hand over more money to the meachanic. All advise is appreciated and welcome.
TVM

drain a capful of oil out and see if its blackish with burned smell

Sorry for my ignorance, what would that signify? I would hope that having only done a couple of hundred miles since the gear box oil was changed, it would still be clean.

there are several reasons why reverse can be lost one been burnt clutch pack

Check the oil level make sure its in Drive with the handbrake on, or better still have someone sit in it with thier foot on the brake then check the level, see if its low