Thanks for your reply!
According to a letter written by Elizabeth CHINEY in 1816 to the overseer of Abingdon parish, her children were Elizabeth age 16 (c. 1800, Tiverton), twin boys age 10 and born in April (we think named William and Thomas), James age 8 and born in May, and another boy "in his third year" (not sure if this means age 2 or 3). I think she must have lost another child at least, as Thomas would have been home when the militia disembodied in 1802 until they re-embodied in 1803.
I don't think the James you mention is 'ours' as I can't see why they would wait several years to have him baptised in Berkshire, but not in the family's parish of settlement - Abingdon in North Berks. as far as I know they have no connection to Shinfield/ Reading and there are certainly other CHENEY/ CHEYNEYs in that area.
The age of the James baptised in Portsmouth is about right, and there are no other CHENEY/ CHAINEYs on the IGI in this area at this time. I hope someone will have easy access to the transcripts for Portsmouth and Portsea and find some conclusive evidence in the parish registers to connect or disprove the link!