Do you or don't you capitalise a sentence when it is parenthesised and occurring before the full stop of the main sentence? Are them some rules here?

No, I would say you don't (though I have to say that I rarely think parentheses work in fiction anyway).

You do capitalise if the previous sentence has ended. (Though that might seem a little pompous.) Then the full stop goes inside the parenthesis.

At least, that's the rule I've always stuck to.

Emma

<Added>

'Brackety Capitals' - what a gorgeous pair of dactyls! Enough to make Hiawatha dance.

So that's what a dactyl is

I realise now that 'a gorgeous pair of dactyls' sounds like a Carry On line - Kenneth Williams or Syd James.

Emma

As in 'Take a look at those anapests'

Oooooh, that's a bit trochee!

Emma

Keep your spondees on.