Thanks all for the replies - they put my mind at rest.
The tomatoes did begin to look worse, leaves at top of one went very odd but it looked like classic mineral deficiency.
Having googled a lot it seems early blight is most uncommon in the UK anyhow - it's a North American phenomenon.
I also read that mineral deficiencies can be caused by overdoing tomato feed, which apparently gives an inbalance of too much nitrogen.
Anyway, seems to me that you're right, there is too little organic matter.
I've been absent from my plot for 3 years and dug in lots of high potash compost, but it was 'double strength' Sheeps' wool compost and it said don't overuse and don't add any other fertilisers. Seems to me that it wasn't enough organic matter but you can't use more than 1 to 2 (compost to earth ratio). Normally I use their standard wool compost because that goes so well with my soil, but this new one doesn't work for me. Seems to me that there is too much nitrogen. The stems are much thicker than usual and my bush types are much leafier than before.
Anyway, I've pulled a lot of the lower leaves, cleared off the mulch top-dressed with worm castings and some plain old wool compost and put the mulch back on to try and get more humus and moisture into it. Also gave it one tomato feed. We'll see how it goes, but actually they're all sprouting toms now and the Arrarat Flame has amazing trusses (3 inch toms).
I think all's well.
Cheers