Thanks for all your help!
I managed to bridge the Technicolor TG589vn (v3); using information found at the following link:
Having conquered that issue, I am now looking into how to install dd-wrt on the R7800, to enable use of a VPN.
I have successfully sourced the required files, I'm now just ensuring that I don't brick my router!
Having jumped once without looking and got away with it, I am exercising caution at this stage!