well, after fitting a new tensioner and then getting the shop to check my clearances again for me, the noise had still not gone, getting worse.
Took back to shop and quizzed the guy
but was told my exhaust was blowing.
I was convinced that was not an air noise but a metal noise.
Indeed the rattling on engine-braking sounded like the engine was going fall apart.
At first I couldn't find it, but putting hands over exhaust holes proved the point. then moving the downpipe with my foot, I felt silly.
Struggled a bit to loosen the nuts, but replaced with new nuts and
...tightened the exhaust to the engine
So, �100 on a chain tensioner,
and �80 for valve clearances,
three visits to bike shop.
I just needed to tighten two nuts, DOH.
Feels like a different bike now. Sorted.