Sadly mate it's a case of retaking your test, A lot of noise was made at the time in the press including a mechanic for a police force who had the police commissioner for that force writing a letter to DVLA saying that the mechanic wouldn't have been employed by them if he didn't have a bike license and they replied with they have no record of him taking the test and a biker who had the bike removed and HGV added.
Unfortunately the burden of proof is upon you. If you can recall the day, date time and place you took the test and passed it the DVLA will not issue the category. As a precaution I photocopy both sides of my licence whenever I send it back to DVLA. I'm not suggesting that they're incompetent, I'd hazard a guess they have such a workload that they just don't check.
It happened to my ex-husband along with a couple of our mates all around the same time.
They retook their tests as no other option open to them. I seem to remember that there was some talk of the possibility of being able to send a SAR's request to the DVLA. Although, the guys had already done their retests by the time this was suggested. Do I don't know if anyone was successful with SAR.
Needless to say he copied his licence front n back once he'd redone his test. Better safe than sorry I suppose.