has the soil had any nutrients added to it over the last 30 yrs? that could account for the yellowing. as for the pale blue powder, they do suffer from powdery mildew

Why not ask the RHS advisory service they are very helpful.
look at their link below

https://www.rhs.org.uk/advice/profile?pid=568

It looks like lichen, to me. Harmless in itself, but tends to show up more when the stems and bark aren't growing well.. What are those gray lumps on the soil just under the edge of the hedge, though? They look like lumps of fertilizer, lime, or even concrete. Is there recent construction in the area?

Tugbrehil, Many thanks for taking the trouble to reply. Your comments re lichen are very encouraging and give me hope that some of my beautiful laurel shrubs are not on a steady diseased decline.
The grey lumps are part of the concrete anchors that support wooden fence posts that have been in situ for 35 years and support a 4' high wooden fence that surrounds my rear garden.
Do you have any ideas of what would constitute a good feed for the shrubs to give them a helping hand back to good health?
Also, should I leave the lichen alone or clear it from the shrub bases? If so, how?
Thanks again.

No need to remove the lichen, As for feeding, the best thing in the long run is to apply a good rich compost once or twice a year, but that may take a year or two to show full results. Growmore would be a good quick fix to start, with compost to follow as maintenance.
If it went 30 years with no problems, and trouble is just now showing, I would look for recent changes. Was the concrete put in recently, rather than 35 years ago? Or any other recent changes?

Thanks for all your helpful comments. I'm off to get some Growmore and compost. There have been no recent changes and I put the problems down to old age.
George

Could be old age, as you say. I seem to remember mention of hedges more than a hundred years old, but those are all boxwood, beech, and yew, to my recollection. I have no idea how long a cherry laurel will live! :)