I have just purchased a Draytek 2820n router. The specs (and box) state it supports RE-ADSL2/Annex L but this option is not available in the modulation list.

I'm not sure if I need a different firmware or need to enable this via telnet. I am using firmware UK 3.3.3. Any help is appreciated.

As far as I know it isn't a type of modulation that you'd select but somethign that the chipset supports, the modulation will be G.DMT if its an 8Mb line or ADSL2+(G.992.5) if it's ADSL2+

If it's not connecting with the firmware its got on it right now, try a different modem code, 211801 from this page should help:
http://draytek.com/user/SupportDownloadsDetail.php?action=Load&MID=12&ID=305