Dave Arquati wrote:

I'm told that the tidal nature of the Thames has an adverse impact
on the cost of running the services, although I admit to some
trouble understanding this.

How about this:

1. With a given fleet, it's difficult to run a fixed-interval timetable
unless you slow everything down to the speed of the slowest boats, i.e.
the ones that are running against the strongest tides. So either the
timetable is highly variable (not good for attracting customers), or
it's quite slow (ditto).

2. In the upper tidal reaches, above Hammersmith Bridge, the water at
low tide is quite shallow, which means that you have to use small boats,
which limits capacity and therefore revenue. That's one reason why
there are no commuter services upstream from Putney.

These two factors affect revenue rather than costs, but the effect is
the same: the services need a large subsidy to make a profit for the
operators.
--
Richard J.
(to e-mail me, swap uk and yon in address)