On questioning the nice people over at National Rail, I was
told that 'going via Clapham Junction is an invalid route.' -
direct quote.

Eh? Surely it's the *obvious* route! It was also 10 mins quicker
than their Streatham Strangeness and faster than farting
about on the tram.

Any ideas what's wrong with Clapham? (as a changeover, I
mean!)

According to the routeing guide Clapham Junction is not on any permitted
routes between Selhurst and Norbiton. However there appear to be two
different tickets available, CDR NOT LONDON £3.50 and CDR ANY PERMITTED
£5.00. Although no permitted routes pass through London anyway, it seems that
the intention is you need to buy the more expensive ticket to go via Clapham
Junction.

Anyway it is probably better to just buy a Zone 2-6 Travelcard for £3.60
which you definitely can use via Clapham Junction.

Indeed, as I pointed out to the website people who quoted the same
book at me as you did.

At some point in the future, maybe the website will be updated to
include more expensive routes (even maybe with a warning) or notice
that the date is a Saturday. (not difficult, 1 function call)

And maybe I could tick a box 'I'm a commuter and have a montly 1-5
anyway'.

cheers, John