Normally, I'd say that the primary guideline should be matching the signs, but I agree that the Weston Point Expressway is definitely wrong. If we follow that, then someone who knows that it's the A533 that goes over the bridge into Widnes may think that Waze is sending them the wrong way.

You could try naming the slip "A557 > A533 (N)". The instruction will then be "exit left onto A557 towards A533 north". That way it will match the sign, but still be accurate in saying you'll be taking the A533.

I've got a terrible migraine at the moment, so you may want to wait for someone else to provide a second opinion; but I've downlocked it for you anyway.

Re-locked

Sounds like a good idea, at least both numbers are mentioned. Change applied. I've sent an tweet to Halton Council highlighting the mistake, but I'm not holding my breath.