OBDII socket issues

[minesapasty]

A week or so ago the Rear Brake Service light came on, but I was unable to get her into my tame mechanic, he was booked solid for 2 weeks (he is a great spannerer, so it isn't surprising). Anyway, so I persevered until my booked appointment was due. Unfortunately, on Saturday, the "Check Rear Brakes" came on, "Oh bugger I thought" and still a few days to go until my booked appointment, so I went on the hunt to see if I could find another local garage, and they were either too busy ( even though there wasn't a car on any of their ramps), or they were closed. I did a search on t'internet and found a mobile mecanicky bloke, he said that he could fit me in on Sunday (yesterday) morning, so the appointment was booked.
He duly turned up yesterday, with parts in hand and promptly changed the pads and the wear sensor, put the wheels back on and all was good....... until he came to plugging in his box of tricks to reset the service light, it wouldn't connect, the OBDII socket definitely worked when the A/C bloke was doing stuff a while ago, and I would guess when the car has been serviced, my usual mechanic would use it to reset service lights etc, so why would it not work now ?
I don't have a code reader yet, but as it would happen, I have already got one flying itself here via mail order from China or some place where it was made.
Does anyone have any ideas what would stop the blokes computer from connecting to my car, he checked it on his van and it connected straight away, so I would guess that says that his box of tricks is working properly...... any ideas on what to look for, maybe a pin pushed in or something, or would that be too simple ?