I added to my pass for the train from London Marylebone Station to Stratford Upon Avon on July 12 leaving at 8:07am. There is a change of trains at Warwick. I tried several sites including Eurail ( which says seat reservations are optional), then went to GWR which says they don't have seats, and it looks like almost the same train but change at Leamington Spa ( which Eurail doesn't have).

Went to Chiltern Rail which says they don't make seat reservations. Very confused. Does anyone know anything about this? Suggestions please. 

They don't make seat reservations. Means: There are no reserved seats at all.

And as you do see (screenshot), Eurail also has the option to change at Leamington Spa if you do wish to change there.

Suggestion: Go to the station, hop on the next train and have a look for an empty seat.

Thank you so much. Do you have any idea which train station, (given we will only have about 10 minutes to change) Leamington Spa or Warwick would be the easiest and quickest for a change? 

It's really no problem to change in any of them. 

But if you are worrying, Warwick is the smaller (easier) one (in fact very small; you will be bored for about 9 1/2 minutes). Should be: Get off, no walk, wait for the connecting train, get on. 

But normally, you should also already see your connecting train already at Leamington Spa when looking out of the window at platform side.  

Thank you. Yes I am a worrier and a planner. Your answer is very helpful.