I don't know Jessops developer nor film, but:(posted 8536 days ago)1. The size of the grain is mostly determined by the film itself. Use a slower speed film 100/125 or slower for the finest grain.
2. Over exposure and over development increase grain size. So reducing the development time might help.
3. Using a fine grain developer will make the grain less apparent. Most of these, however, reduce speed somewhat, and they dissolve the edges of the grain clumps so the look is softer. You may or may not like this.
3.