Topic

Like it says on the tin, the windows works when it wants to and doesn't work when it cant be bothered.

I have read a few places that it might be the replays in the BCU?

Is this likely?

Is it an easy swap

You need to find out what you have depending on your Vin as the systems are different, I know if you have an SCU they are soldered in.

Personally I would start by temporarily wiring a bulb in parallel with the window circuit so that next time it fails you can see if its the relay/ECU that is failing to provide an output or if its' the motor itself/ wring to the door.