"Paul Corfield" wrote

1. Someone asks on the unofficial Oyster Rail website. Mike Whittaker,
the owner of said site, is pretty clued up on the workings of Oyster
and answers queries pretty quickly.

http://www.oyster-rail.org.uk/peak-off-peak-and-caps/

And here is what seems to be an answer to the original poster's question
(no reference to caps)

http://www.oyster-rail.org.uk/peak-o...#comment-18954

.. There are three ways to do this simply, depending on whether you value
time or money. The fastest and most expensive costs £8.60 each way. Just
take the Northern line to Euston and any London Midland train to Watford
Junction.

The second way involves splitting the journey at Euston to take advantage of
the off-peak fares in the ‘wrong’ direction on the Euston to Watford line.
If you wait at least 40 minutes between touching out of the tube and
touching in at the NR station then the fare comes down to £2.10+£4.50 =
£6.60 each way. On the way back you only have to wait at least 20 minutes
between touch out at the NR station and touch in on the tube. You could
touch in on a bus and then get straight back off again outside Euston
station which would cost £1.40 but you wouldn’t have to wait.

The third way involves taking the Bakerloo line to Willesden Junction and
touching out and back in again before taking the London Overground to
Watford Junction. This would cost £2.80+£1.60 = £4.40 each way, but would be
quite a bit slower

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Mike D