In message , Serendipity
writes

I asked at two of our local Garden Centres for something I
could use to attack leaf miner. They both gave me the same
answer. "There is nothing now avaialable."

Garden centres increasingly seem these days to be nothing more than
box-shifters (or more accurately plant-shifters).

Don't know if its still available, but Dimethoate used to the accepted
control. This is a systemic insecticide and is diluted and the soil
beneath the shrub/tree treated at a time when you expect no rain. Dig
between four to eight holes 6" deep holes around the tree below the
outer span of the foliage (the number of holes depending on the diameter
of the span - the bigger it is, the more holes you dig). And fill the
holes with the drench, allow the drench to soak in and refill with soil.
Don't overdo it.

Do it once a year when the shrub/tree is in leaf (early to mid May).

If your chosen garden centre looks blank when you ask for Dimethoate
(likely these days), search the shelves are read the tin/container of
the various systemic insecticides for the name Dimethoate - and dilute
according to the tin's directions (it may not mention leaf miner).

Don't overdo it. You could do more damage than the leaf miner does :-)

--
Tony Morgan