Good to see I'm not the only one having issues getting the tax class updated. I've been in the post office 3 times with my V5 trying to get it changed to historic. Each time they scan it and it errors on them.
Phoned the DVLA last night and they say all looks fine to them and can't see any reason that it shouldn't have gone through.
They told me to fill in the new tax class on the tax book as 'Historic', sign it and send it off with the insurance certificate and V112.
The only thing I can think of that might be making it fail is it has been without MOT for 2 years but the way it is worded it appears it requires either a valid mot OR V112.
Sent it all off today so the wait has started.