I've upgraded my trust 2820 with the out-of-data firmware to allow for DHCP option 61 with a 2925. Pleased to say that it is all working (did have to clone my MAC address even though others have reported it isn't necessary) . As before I'm using a HG612 as my vdsl modem.
The 2820 is now off to my Dads to replace his older 2800. Hopefully it will play better on his long line now that I have loaded on the long-line firmware.