Thank you for your replies.

The spots are small (less than a centimetre diameter) on my bay tree and grow a little larger on the Sycamore (maybe nearly 1cm) and look as though they do grow brown (not on my bay tree yet). I'll check out texture over the weekend.

I can certainly get rubbing and I'll speak to someone at a nursery to see what they suggest - I certainly do love me cookin' with fresh herbs.

I just hope they don't spread to my olive trees.

Hopefully the Sycamore will at some point get chopped down - working on that presently, as they are stopping my tan in this lovely weather!