[wasserkuhl]

Best way is to dismantle the doors and then remove the handles and inspect the action.

I suspect that as you are now in the UK you may have some corrosion within the door handle itself.

You can test the action of the handle when its removed if it feels free, then you need to look within the door cavity for the rods and mechanism that actions the door latch (from the door trigger handle it pushes the mechanism into action......)it may be bound up or even fouling on something. The first time I re assembled my doors I found that one was binding up within the door cavity......

Good Luck.