Discussion: engineering solution

Gaffa tape it.

That's pretty much what he suggested doing! Wrap tape round my leg to hold it together.

Which ligament? Where is it exactly?

Just having trouble visualising how you could damage it....

The only one I can see doing what you describe is anterior tibiofibula ligament, but not quite sure how you'd tear that one!?

Yep that's the one. Torn seems like strong word to use but must have ripped some of the fibres for it to have become a bit 'looser'.
Physio wasn't sure either as the way I twisted it isn't the normal way of damaging it.