Hi,
I've also got DTX and passed my test today. So got home and derestricted it straight away
From the back of the speedo follow the wire to where it clips into the loom. Unclip it and see which of the wires coming from the bike doesn't connect to anything. Pull that out of the plastic clip, earth to the frame and you're done. The wire on mine was black and greeny/blue, guess yours will be the same.