[Bushey - Euston(Overground)]

Tells me 4.50

Page 8 of the document says that Bushey is in zone 8

and page 21 of the document says that a zone 1-8 single is 5.50

There seems to bee some special 'Overground Only' fares. If one extends
the journey to anywhere else in Z1 it's 5.50.

Adding the better off-peak times it might be a good idea to break a
journey at Euston.