Welcome to the world of the seized up trigger block.

I recently had a badly stuck Mercury and my Mk6 bought new and a Mk1 Airporters were also stuck fast.
I used repeated cycles of heating and cooling over a couple of days and tried to get easing oil to the threads from inside the cocking slot leaving it to soak overnight.
I only had the wife's hairdryer and was never sure whether the heat would conduct to the block's thread. The block does turn anti clockwise to loosen and a rod through that hole is what others use to turn it.
Give it time and every so often clamp your trigger block in the vice and try to turn the whole action with both hands. Just when I was beginning to despair with the Mercury I tried that and it popped straight off.

