Thank you ViV and silvery,
The gap between George and Mary is neatly filled by Martha who was baptised 20 Aug 1710. Martha married in 1732 so she was presumably not considered at the time, being both female and married.
John and George were also married by the time Simon died but Mary didn't marry until 1735.
I am guessing the inventory was done by the same person who wrote the letter as the inventory is mentioned in the letter. His name looks like John Pope. I haven't found anything on him yet.
Paul