Found some more minor stuff for Isaac Pickstock. Ancestry has
UK, British Army Muster Books and Pay Lists, 1812-1817 and there are two entries in here for him:
Name: Isaac Pigstock
Start of Muster: 25 Dec 1811
End of Muster: 24 Mar 1812
Stationed or Muster Place: Maidstone
Regiment or Unit: 2nd Veteran Battalion
Piece Number: 11119
Name: Isaac Pigslock
Start of Muster: 25 Dec 1814
End of Muster: 24 Jan 1815
Regiment or Unit: 2nd Veteran Battalion
Piece Number: 11119
Now, there are a couple of things to note here. Firstly I can't blame Ancestry for getting the Pickstock names incorrect - the writing has more style than legibility and I can't swear that the originals do read Pickstock but it's virtually impossible that they're not him.
Similarly, I'm reasonably clear that "Maidstone" is actually "Madeira"!
Also - why the gap? Well, the filmstrip consists of 768 images of what I
think is one physical book, TNA ref WO 12/11119. The book seems to consist of a number of quarterly muster-books / pay-lists (not sure if there is a difference) for the 2nd Veterans, all bound together in date order(?). It's my belief that Ancestry have only indexed the first and last pay-lists in the book - I easily found Isaac in the next quarter (sort of Q2 for 1812) but the page was not indexed, and neither are others in the middle of the piece that I've checked. So, I think he would be in every quarter from (sort of) Q1 of 1812 to the last list, which is actually for just one month, ending 24 January 1815. Why that date? Because 20 January 1815 was when 2nd Royal Veteran Battalion was disbanded (I'm not looking for exact matches to within a day).
So that seemed to take Isaac up to January 1815 - which was not quite what I expected, as I thought that the RMA School was for orphans and Charlotte had started there 6 January 1815, while her father turns out to be very much alive. Or at least, being paid as if he was.
However,
http://www.richardgilbert.ca/achart/public_html/articles/york/history.html is about the RMA and right at the bottom of the page is a link to miscellaneous correspondence from researchers - follow that and it seems that there are codes in the Admission Register that indicate whether the father is dead or alive. In other words, it seems like Charlotte need not have been an orphan on admittance after all. So Isaac may well have died
after 1815.
What I intend to do is get down to Kew and have a look at the originals of the Admission registers to see what might be there, that isn't in FindMyPast - such as the code for whether Isaac is alive or not. There's also correspondence ledgers for the school and who knows what might be in them. Not sure when I can get but hopefully not too far off - Richard is definitely one of my Pickstocks, so this is bugging me as well!