Tim wrote:
I have a zone 1-3 travelcard loaded on my Oyster card, and want to
travel from Clapham Junction to Kew Gardens. This involves getting a
train to Richmond then changing for Kew Gardens.

However Richmond is in zone 4, which I have always assumed means
having to buy an extension ticket (for something over a pound). This
seems like an unreasonable expense & hassle, since I don't want to go
to zone 4 or to Richmond, it just happens to a necessary one-stop
detour on the way to Kew

It's not "necessary", it's just that you choose to go that way, and have
to pay the fare for that route. Alternatives covered by your Z1-3
Travelcard would be:
- by South Central to Victoria, then District.
- by SWT to Kew Bridge, then a 65 or 391 bus (or walk) to Kew Gardens.
- via Silverlink to West Brompton, then District line changing at
Earl's Court.

--
Richard J.
(to e-mail me, swap uk and yon in address)