Actually, I first noticed the mottling on the leaves (on the fruit I only noticed it yesterday) when it got v hot here (for the UK!) a few weeks back - daytime C temps were in the low/mid 30s, nighttime not much below 20. Last few weeks has been different, daytime more like early 20s, nightime low teens - and it's been quite damp, which is why I've been on blight alert, but this didn't quite seem to fit that bill.
Watering-wise, generally every 2/3 days when it's been dry - a bit more often when it's been hot. Tomato feed once per week.
That particular plant does sit in the sunniest spot of the garden, facing due south. The odd thing is the majority of the leaves on the plant look normal and overall the plant seems healthy.
I took off one branch and tomato to get a decent photo for you but do you think I should remove any other affected leaves/fruit?
To be honest, good to hear it's more likely to be stress as my fear was it would be something more contagious!