I had this happen to my Sigma 150 and I had it replaced after the Memorial Blat by CC South after a chat.
Hopefully the following should give you an idea of what the Duratec R400 Radiator system should look like in a Sigma 😳
The 1st Pic shows the tubing that comes FROM the engine block to the Expansion Tank as does the 2nd (but from a different perspective) , there WAS a 'T' piece that joined the self bleed system to the tube from the engine and then into the Expansion Tank (if you see what I mean)
The 3rd Pic just shows the Expansion tank, with the tubes coming in and going out. IIRRC the large tube that sits bottom right in the pic (assuming that your viewing from the same angle took the picture at is not modified in any way.
I can't remember off hand quite how they filled the system, but I do remember that they undid the bleed screw at the top of the R400 radiator filled the system, let it settle and then filled it again until coolant dribbled out the bleed screw hole and did the bleed screw up again - there was also something done with the heater, but I didn't pay attention to it (sorry).
Hope that's of some help