Logically, the 4th would sit somewhere around junction 10 (A3) or
junction
11 (M3).

Given that the south-west quarter of the M25 is the most intensively used,
it's surprising that it wasn't the first to get a service area. It's also
surprising that there aren't signs at each junction on the M25 to the
nearest off-motorway services where you could at least get a cup of coffee
and a Mars Bar, and go to the loo.

Good point - of course it couldn't work at J11 (motorway:motorway only), but
any obvious reason they couldn't do something like that for the A3 junction?