Could someone help me identify what has occurred on the leaves of my pieris please.

q-2tip

Papi Jo

Your pieris might be a victim to the infamous andromeda lace bug. See this discussion.

Could you please post some photos of the underside of the leaves?

bédé

Does it rub off with a wet finger?  It might be algae.

Igrowfromseed

My pieris has always looked like yours during the summer. Thought it was red spider mite?

Fairygirl

Here's some info on the lacebug 
https://www.rhs.org.uk/advice/profile?pid=663

It's most common in the south of England, but it may well be spreading further. Never seen it here, but we're a long way from the south of England. 

ERICS MUM

My pieris has this every year, just on a few leaves.  I haven’t seen any long term damage from it though.  

I feed it with an ericaceous liquid food a couple of times during the summer, perhaps this gives it the strength to resist disease ?

AaronBil

I spray mine immediately after flowering to reduce infestation. Feeding also recommended.

bédé

Igrow, Your pics do look more like spider mite, but difficult to be sure without a magifying glass.  There will be fine cobwebbing on the underside and you can see the little mites moving about.  (Called two spot mite in USA, a better name, as they are not red.)