Peter Masson wrote:
As I read the leaflet you can use any ODTC, including an out-boundary one,
for an evening peak journey entirely within the Travelcard zones.

Yes, and there is no way they can ever change this. You can always use
an off-peak travelcard, irrespective of the origin point, on any train
for any journey within the travelcard zones.

So yes you CAN use a Luton or Bedford to London all zones off peak
travelcard between London and Elstree & Borehamwood, without
restriction, and there is no way they can ever stop you.