Thank you – This was bang on and apples to model SI-10T as well (I put the wrong model), I ended up paying a former Britannia engineer who works for another firm £100 to do it. Basically, it just needed a little more upward pressure and was caught a little on a side bracket as you lift up (hence the resistance), The trick was to “wobble” it forward once lifted up, I was being a bit t paranoid that I would break the glass or something inside if too heavy-handed.
Now I know what to do next time a regulator/thermostat/simmerstat goes so £100 is well spent.