The guy's a twit and is clearly trying to pull a fast one.
You have the new keeper supplement. Stand your ground and get on to the DVLA. They're normally pretty accepting that they screw up, as they seem to do it on a regular basis.
If it were me, I'd be down at the local office on Monday. They seem to take things rather more seriously if you show up in person - I've done it quite a lot, as the local DVLA office is about 1/2 a mile down the road from home.