[paul cotter (OP)]

When I need to return to a post to perform a correction, the maths symbols are no longer available. Any solution to this?

[evan_au]

Try using the Actions > Modify option, rather than "Back".

[Halc]

Yes, action->modify gets you into a thing with the symbols.
Also, highlighting a subset of a post and then clicking 'quote selected' puts you in quick-reply, as does just typing in the box at the bottom of the page. Quick-reply doesn't have symbols, but if you click 'preview' button below your post, you end up in the not-so-quick editor with the symbols.