Is it possible to run Annex M on Vigor 2820s?

The specs sheet say they do, but I've just had our line upgraded and I've set the Annex M option under the Modulation drop down - but no such luck.

The modem itself has Annex A printed onto it.

Does this mean Annex M is not possible? My ISP is EntaNet

It depends whether your account has Annex M enabled on it, the 2820 definitely does do Annex M and all you'd normally need to do is select that modulation from the list assuming your line / account supports that modulation.
This modem code tends to work best with Annex M: ftp://ftp.draytek.com/Vigor2820/Firmware/V3.3.5.1/annexA/2820V3.3.5.1A2471201/

HI Voodle - that's worked a treat. Cheers