Doctor put me on a course of Ibuprofen for a week which helped a lot.
It's not gone totally but it's a lot better (9 months since it started).
I triggered it by using one of those Powerball wrist strentheners, ironically I've seen then suggested as a therapy for Tennis Elbow. When I first did it, it felt like someone had a soldering iron on my elbow, then it migrated into my forearm. I could honk on my handlebars all day and yet couldn't pick up a pencil. I think it should be called "Kettle Elbow" as that's where most people experience most trouble, picking up a kettle.
Using a PC mouse doesn't help with recovery, swap mouse hands.