Thunderbird Super Coupe auto (AOD) third gear seems to have turned into neutral !

Hi I live in the uk and was driving my tbird (3.8l supercharged v6) doing about 70 mph in the overdrive gear (auto) and all of a sudden the gearchange to 3rd was a bit of a jolt and I lost 3rd, its now neutral. The car still drives fine in 1st and into 2nd but when it goeas for 3rd there is nothing there, it just revs. I have tried in the normal drive and still no third. Reverse etc everything else works but im stumped as im here in the uk. I have not lost fluid in he box as its still topped up correctly.
i have heard of a tv cable to check but have not checked yet. Am I in need of a new gearbox?