SCART pinout (female connector seen from the front). Image is taken from Wikipedia.
After a visit to Wikipedia (source of all wisdom) and a bit of poking around with a multimeter, I found a 2.5v signal from the DVD player to the TV on SCART pin 16, even when the DVD player was turned off. This should tell the TV to expect RGB (rather than composite video) from the DVD player but appears to have a negative side-effect. Snipping pin 16 in the SCART cable has cured the problem.