I have a zone 2-3 weekly travelcard but sometimes need to go to
Watford Jn on the London Overground at around 8am. I can get to
Willesden Junction on my travelcard, where I change onto the London
Overground - in order to change I don't need to touch in or out.

If I'd bought a single PAYG from WIllesden this would have cost me £1
as it is a local journey away from Euston (right?) - so how come I am
being charged £3 as an 'extension fare' for the same trip? Have I done
something wrong? Is there anyway I can avoid this (maybe touch out and
back in at Willesden?)

Anyone any idea what the 'extension fare' to Watford would be if I
owned a 2-6 travelcard instead?