Thanks, Mike. Not to labour the point but today I found myself in west London with a travelcard and some time to spare so I took the journey myself to see how reality matched up to the theory.
I only had time to take the journey once in each direction: Wembley Park to Kew Gardens via Kilburn and Brondesbury took 34m45s; Kew Gardens to Wembley Park via West Hampstead took 38m13s.
Approaching Wembley Park we did have to wait for a terminating train to clear the platform so I think the northbound trip was about a minute longer than it should be. Apart from that I had good journeys both ways.
I’m sure that Stephen now has more information that he wanted or needed but hopefully some of it is useful 🙂