So just an update on this to conclude the tale.
Went back to Halfords store and their gauge showed that it was still pressurised and didn't need topping up.
They did however suggest an independent air con specialist.
Booked it in with Cool It Automotives at Martinstown, Dorset. He put the proper multi pipe gauges on and it showed low, so drained, filled with air and then recharged coolant - all for £50.
He thought that Halfords gauge probably showed OK as it was a hot day, so all the pipework would be hot under pressure when they should be cold.
And now it all works fine, and the keys are freezing when I remove them from the ignition.
So the lesson is, go to an independent specialists first and avoid Halfords
2012 B-Max 1.5 TDCI Zetec