I thought I would leave this on as it might be useful for other members even though I have now called Flythomascook.
After
Googling - I read a number of reports (particularly long-haul) where is seemed you would be removed from the list for the return leg if you didn't fly on the first leg so I decided to call Flythomascook.
The young lady did ask me
where I was flying to - and then said - "let me check" - she then came back after a few seconds and said "there will not be a problem - the gate will close an hour before take off and you will be down as a no show - but will still be included on the register return leg" - so fingers crossed - but it does seem to vary from airline to airline - so be careful
Ian