You need to remove the metering unit from the base (3 x torx bolts on top - will need to remove all lines first).
You can then turn it over and you'll see the plunger in the middle - if its proper stuck which it sounds like it is, it will be sticking out which works in your favour - you can then try and get it pulled out - id try and get something like a small amount of lead and then put this round the plunger then use some grips on that to try and wriggle it free (the lead will take the pressure and hopefully avoid marking the plunger).

There is a spring behind the plunger so keep an eye out for that in case it disappears into the abyss

Once its out, you can review the bore, and try clean it out using fresh petrol and gentle agitation with a brush or similar.
The end result you want is for that plunger to move up and down with minimal resistance and no manual force.

Plunger:



Bore where the plunger will be stuck: