Well to update how I am getting on, just done my second oil transmission oil change using the Carlube ATF-U (after three changes using Millers ATF and around 200 miles after my first change on ATF-U) and sadly not a lot of improvement. Gear changes between first and second are still very harsh at times, giving a real kick sometimes, and sometimes can feel slipping when changing from second to third. It seems worse when the engine is cold. I left the battery disconnected for two days to try and reset the transmission PCM but seemed to make no difference. On the latest change the oil came out fairly black again, which noticeable black particles in it, presumably from the clutch plates in the torque converter or the clutch packs? Will do another 100 miles or so and a final oil change to see if things improve, but not too optimistic. Other than the transmission issues the engine is really strong and smooth and rides lovely once up to speed and the gear changes between third and fourth seem fine. It is just around town when the transmission issues make for a less than relaxing drive. Starting to consider parting with her before the gearbox dies altogether, but will do one more change before making my mind up.