2013 vRS - front offside brake keeps sticking, yet releases with complete ease with even gentle prising off. The caliper moves back in freely, the pads are not stick in the guides (freshly copper-greased and replaced due to sticking). nearside - no issue. Anyone come across this and any ideas?

Something is stopping the retraction. My only experience of a similar situation was in the late 70s with an Austin Maxi. The master cylinder was corroded and sticking due to water ingress around the vacuum tank. this meant that slight pressure was always applied to the brakes. I do not know if such an analogy can be applied to modern braking systems. Maybe change the brake fluid as a starter? 

It may be a blocked brake hose or line. I have seen this before on older cars. Try to change the brake hose and blow through the brake line with compressed air.

I had a case on my old beamer where the brake line had become kinked and working like a non return valve

That's a possibility I suppose (not the master cylinder as it is only one caliper) but there is no sign of the hose being damaged and why is intermittent and easy to free with a prise of the caliper? As I said never come across this before

3 hours ago, goath said:

That's a possibility I suppose (not the master cylinder as it is only one caliper) but there is no sign of the hose being damaged and why is intermittent and easy to free with a prise of the caliper? As I said never come across this before

Sometimes the hose breaks up inside and the debris can act like a one way valve. Rare these days but you never know. Now you won't want to hear this, but my money is on the ABS unit. There are modulator valves for each wheel and it's a good bet one of them is sticking.