I would like to have this clarified 100% as I've heard different
stories, namely that you can combine two seasons and now the messages
that say you can't. If you can't, the next best option is to get a
Travelcard up to Finsbury Park and a NR season from there - because
almost all trains stop there.

When I looked at the Z1-6 and Hadley Wood to Hatfield option, with an
annual ticket, the savings were significant - nearly £300 (a search on
uk.railway will give the exact figure, as I've lost my paperwork).
However, if I'd have to get a slow train that stops at Hadley Wood then
it's not quite so appealing.

Finally, even if it DIDN'T work out that much cheaper, the extra
convenience of having an Oyster is obvious. In fact, it might even be
worth paying a little more for (in my opinion).

Jonathan