How hard can I hack this back?

I suspect the heavy clay snorky.
I have always grown them in shade

 

Ah maybe it’s because mine are all in full sun

 

Aucuba blackening / RHS Gardening

The RHS seems to put leaf-blackening down to wet soil. My Aucuba picturata died a long, lingering death on a well-drained, shady bank

 

Yes I do think wet soil is the problem in Snorky's garden despite growing in sun. Perhaps the clay soil is more saturated than appears on the surface??
I moved mine a few years back as it always appeared below par.......now in a semi shaded spot it thrives. One of those plants that should be tried elsewhere in the garden perhaps before giving up on
In full sun in poor soil I suspect leaf burn will be a problem too ....

 

I suspect with many plants, it's their inability to form good roots into the clay that is the problem, @shiney. Especially when they've been raised in peat or peat substitute which resists re-wetting if it's dried out at any stage. I used to have a real struggle to convince clients that pot-grown plants needed slow, careful watering for anything up to 2 years after planting into unimproved clay.

 

Certainly I can confirm you can hack back the ordinary green Laurel as hard as you like. I took a chainsaw to 1 of mine...not a favourite but massive job to totally get rid of it. The branches had diameters of about 10 inches so not quite a job for secateurs altho knew if dealing with your average shrub, it doesn't look too clever cutting Laurel leaves in half when pruning. It took a while but new buds eventually broke on the cut branches and 2 years on the brutal pruning has done the shrub wonders. Burnt the pruned wood in the log burner and it burnt very well...bonus!

 

Ah yes, but you're a gardener