Received this reply from Thetford regarding the flush tank adaptation:-
Thank you for your enquiry with regards to the C250 drain tube.According to our records your toilet does not require the modification.
Your toilet has been fitted with a reinforced drain tube so swapping it
over to the new style is not necessary
So unless Bailey have been using a secret cache of antique bogs that they have found in a cupboard under the stairs on some vans then us Orion owners it would seem are in the clear.