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Driver overstayed this car park, as they got into difficulties in another shop nearby. We did shop in the store afterwards, but no longer have the receipt.

The cameras look to have been installed some time in March.

Here's the front: ECP Invoice front
Back: ECP Invoice back
Signage: This is the only sign

I was a user/helper a few years back when Pepipoo was still a thing. Hopefully, I can still fight this one now.

Edited to add that although this is dated the 16th, it didn't arrive until 22nd. Is this a new frightener they use to get the nervous to cought up?

Welcome. Your first starting point should be to contact Morrisons, explaining that the driver was a genuine customer. Morrisons anecdotally have a decent track record for intervening in these matters, and even have a dedicated form on their website that can be completed: https://www.morrisons.com/help/form/contact-us/car-park/car-parking-charges/i-have-a-car-parking-charge.

If you don't have any luck with them, then ECP charges can still be beaten, but it can be a somewhat protracted process - basically you appeal first to ECP and then to POPLA. If both of those are unsuccessful, you then wait for them to initiate court proceedings. ECP usually use DCB Legal who, if you defend the case, nearly always discontinue before the hearing.

It does state that other signs are present.

There certainly were as of March of this year: Here for example

I'm intending to go into Morrisons at the weekend and see if I can curtail the process.

You've just been given excellent advice on how to deal with Morrisons (Plan A). Going into the store is highly unlikely to resolve this. Use the link provided above by @DWMB2 and submit it that way. Invariably, using that link gets this cancelled for you:

https://www.morrisons.com/help/form/contact-us/car-park/car-parking-charges/i-have-a-car-parking-charge.