Thanks for the quick reply!
I'm travelling from Gatwick to Victoria then back again a few days later
so I'll go for a priv ticket. I had a quick look at the 'travel facilities
guides' on the ATOC site (incidentally, many thanks) and it doesn't
mention anything about the Gatwick Express although the Heathrow Express
does get a mention. I take it they are run by the same TOC?
Ideally I'd like to use one of my boxes but a PT is better than nothing!

--

The key word is 'safeguarded'.
Anyone who worked for BR, or was entitled to BR privilege travel facilities
before privatistation
is entitled to keep them under TUPE rules.

Gatwick Express used to be part of BR, and so all BR privilege travel
facilities are valid
(I can confirm this, as I have used them).
Heathrow Express is a new operation which started after privatisation and
was never part of BR,
therefore safeguarded travel privileges are not vaild.

--
Peter