May as well ask this here as I'm visiting Harry Potter at some point this year..
I see that Watford Junction lies just outside Boundary Zone 9 according to the latest tube map. If I have an Amersham Und - Zones 1-6 (AAA LDN Zones 7-9) Daily Travelcard, am I correct to assume this would not be valid all the way to Watford Junction and that a Boundary 6 - Watford Junction extension would also be required (they don't seem to have Boundary Zone 9 extensions)? If so, I will just buy a standard 1-6 Travelcard and Boundary 6 extension instead.
There is a special Z1-Z9+WFJ Day Travelcard ticket that you can buy from all London Underground / Docklands Light Railway / Croydon Tram TVMs as well as National Rail TVMs with a "tickets from another station" feature (by selecting from Watford Junction to London Z1 - Z6). You should also be able to get it from all competent National Rail ticket offices.
These are the Day Travelcards that are available.
Z1-Z6 = £12.30
Z1-Z9 = £13.10
Z1-Z9+WFJ = £17.50
These are the boundary zone extensions available.
BZ6-WFJ = £4.80 (Return)
BZ6-WFN = £4.50 (Return)
BZ6-GSN = £4.50 (Return)
So a Z1-Z9 Day Travelcard plus a BZ6 to Watford North or a BZ6 to Garston (both of which are strangely cheaper than a BZ6 to Watford Junction ticket) would bring the total to £17.60. So your best and cheapest option would be to buy the Z1-Z9+WFJ Day Travelcard ticket for £17.50.
However if you are not travelling to anywhere else in Z7 / Z8 / Z9 on the day (for example Amersham or Chesham etc) than your cheapest option would indeed be to buy a Z1-Z6 Day Travelcard for £12.30 and then a BZ6 to Watford North or BZ6 to Garston (again as they are cheaper than BZ6 to Watford Junction tickets) which would bring your total to £16.80.