WARNING!!!!
Much of the JP’s chassis is, I believe, alloy and brick acid will immediately fizz and start eating into it with quite a violent reaction.
I would suggest getting the area around the cotter good and hot with a hot air gun or very carefully with a blow torch. Then putting the nut back on the cotter and giving it a firm hit. The cotters are, I believe, a standard cycle pedal crank part so are cheap and expendable.
At the risk of forum disloyalty, there are a couple of JP experts on the Old Mower Club who have probably encountered the issue and may know the answer.

Let us know what you find out, Charlie.

It may help if you could post a photo of the underside of the frame to show the other end of the fixing that you’ve removed. If the thread is loose in the hole it’s more likely to be a two-piece collet with a bolt going through them to draw them together and clamp them on the shaft. A sharp tap on the thread (once you’ve protected it by winding the nut back on) should push the bolt out of the lower collet, with the added advantage that if I’m wrong (and Angus is right) it will do the same job!

I’ve been doing a little homework since this morning and it appears that the pin with the thread on is inserted into a blind hole in the casting, the shaft is then fed through it’s hole then a sleeve is put onto the threaded part of the pin and everything is drawn up tight with the nut.

You may have loosened the one that you’ve been working on sufficiently but you will have to do the same with all of the others to get the side plate to move enough to get the roller out. A shame that they are in blind holes and you can only get to one side.