Once again a lot learnt about syntax and Variables (Remembering tomorrow the hard part). Finally went the route of setting a different variable for each Custom Data Field then copying that to the new room selected, Update the Room field in the booking Entity, Clear all the Old Room Field and finally Null set all the Variables ready for the next Transfer. Now need to tidy up the numerous unused Rules I have written and discarded and all the test messages.
And it works!
Finally think I am in a position to implement the POS as it now fits the business model.
Just set up the Printers, terminals, inventory etc etc. Spent far too long on the Hotel side but will be worth it.
Many thanks (probably not for the last time) @JTRTech.