Hi Jacko77, thanks for your post which is interesting. It does appear to me like you need to go into a clinic and be physically assessed. Your observations about hammer curls being less of an issue is a useful on as there is very little demand on the forearm flexor muscles when doing a hammer curl compared with a normal barbell curl. This is as the forearm is in a neutral position as opposed to a supinated one.

I would suggest first assessing the proximal (near elbow) radio ulnar joints & the nerve tracts from neck to hand (including the neck itself) to clear other areas referring symptoms to the medial forearm area. If cleared then a muscle assessment to check strength and possible a diagnostic ultrasound if things are positive for those tissues.

Do post a photo comparing your forearms flexed if you can as that might show some observable signs between the two sides.