To give some update on this I managed to remove the suspect part.
1. Seats out
2. Remove the padded gearbox cover
3. Remove all screws
4. Discover that the transmission tunnel sits over the gearbox cover!!!!????
5. Peel back leather glued to the transmission tunnel
6. Remove all screws and remove transmission tunnel
7. Climb into car and attempt to raise gearbox cover
8. Use large screwdriver and hands to pull and lever the cover away from the glue/sealant, climb back into car and lift the rear of the cover.
9. Work the cover back and discover tha it can't be lifted over the brake lever and is restricted by the upturned edge at the front fouling against the bottom of the heater box.
10. Work through the gaps to remove the 4 bolts holding the remote gear lever case to the top of the gearbox.
11. Remove remote spilling oil on the carpet that would be under the seat

(I should have kept it tilted upwards and I should have had some rag under it as I removed it)
12. Remove carpet and wash out the oil with copious quantities of detergent - hang carpet to dry.
13. Dismantle remote case, photograph and email description and photos to Vitesse
14. Head back up the mountain for a well earned beer.
After a few emails backwards and forwards to Vitesse it is starting to look as though the problem may not be in the remote gear lever extension even though it is one of the earlier versions (2011) without welding. It is not yet definite but the problem may be in the selector mechanism of the gearbox itself. If this is the case it is an engine and gearbox out job. I will know for sure in the next day or so.
Vitesse tell me that if this is the case it is extremely unfortunate as it would be the only case of gearbox failure in over 6,000 gearboxes that they have supplied.
I have to say that Vitesse have been fantastic in all this with prompt and helpful replies to my emails. I wish all businesses were as correct and helpful. Judging by this I can only say that anyone considering a gearbox conversion for an MG or one of their other products, including the Morgan Plus 4 automatic conversion, should have no concerns about aftersales support.
https://www.vitesse-ltd.com/blogs/news/110594119-automatic-for-the-people-morgan-plus-4-automatic