You can buy an aerosol can now that'll freeze the component when sprayed on, Rob, my hydraulic pipe man uses it to free pipes off.
I've had to resort to wrapping the timing belt off a car round the tap area of the cylinder to hold it in a vice before now with a pin through the back block and a piece of pipe as a lever.
On this one I indexed the male threads one tooth forwards on the back block and the trigger housing was still plenty tight enough on reassembly.
All the best Mick