Cheapest way to take a car through the Channel Tunnel

It seems you are right - the ferry is cheaper. Last time I looked it seemed that the train would be a bit cheaper if using the round trip offer. Still the train includes 9 passengers while the ferry only includes 4, so for many passenger the train will be more competitive.

I am travelling by road to Turkey next week from Northern Scotland. The ferry I've booked, DFDS, is £29 one way (Dover-Dunkirk). The train at a similar time was around £90. There are only 3 of us going.
No contest really, though the choices are good.

They can blacklist you if you try to use day trips as a cheaper option, and may cancel the 2nd day trip if tat is your 'return'.