Thankfully a simple remedy and cause and my suspicion was right.
Wiped the cases on a cloth thoroughly before firing today both with and without suppressor. No pressure signs at all and easy bolt lift. Made no other changes (didn't even clean it - other than a boresnake through to ensure no residual oil in bore).
I now realise I used Lee reloading lube (white paste type stuff) when working up the load which was easy to wipe off. I have since switched to RCBS lube on a pad. It's a lot oilier than the Lee lube and obviously takes more than a wipe with paper towels after loading to remove the lubricity.
I'll starting wet tumbling for a short time with some dish washing liquid after FL sizing now to ensure all lube is completely off.
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