Free parking (in that there is no charge for parking) provided certain other conditions are met isn’t necessarily incorrect. The sign has to be read in context.
And when one of those 'conditions' involves spending money, so that you to be allowed to park without incurring a charge?
Have you been charged for the parking?
What's that got to do with the price of fish?
I'm asking about the validity of the lack of info in the signage, not help with a ticket.
and what did the store manager say when you queried it with them?
you did didn't you?
Dunno if it was the store manager but the person I asked said that they had a 10m in grace period before being charged. I replied that the sign said there was a 90 minute period before being charged. 
NO. ... about your issue with having to provide a receipt to enter your VRM.
Shrugged and looked blank. So I went and bought a pint of milk so that I could get my 'Free Parking'.
What's that got to do with the price of fish?
His question would appear to be hypothetical, he's not suggesting you've got a ticket.
One of your apparent issues with the set up here is that you believe the free parking is not 'free' if you have to make a purchase:
The point I'm trying to make is that it's not free if you need to purchase something
With that in mind, Southpaw82's question seems entirely relevant - in the situation you describe, would a driver have been charged for parking when making a purchase?
I'm conscious we're now 32 replies in, and it's still not clear whether the shop are happy to provide a receipt to use the terminal for people who visit without making a purchase - if they are, much of this debate would probably be moot.
What's that got to do with the price of fish?
Lots. Maybe answer questions instead of gobbing off at people because you perceive it as not fitting your agenda.
I'm conscious we're now 32 replies in, and it's still not clear whether the shop are happy to provide a receipt to use the terminal for people who visit without making a purchase - if they are, much of this debate would probably be moot.
As a Lidl customer in many of their stores my experience is that if one does not make a purchase for any reason the cashier will provide a "receipt" to use at the terminal to log the vehicle registration. Usually there are many receipts around the terminals which can be used if no is purchase made.
The purpose of using these "registration" terminals is to discourage parking by those who do not use the store. At my local Lidl it is common to see people park and then leave the premises to shop elsewhere then return and enter Lidls presumably to purchase or use the terminal. Lidl seem to tolerate this.
Who makes up the rules as they go along?
The sign says "Customer Parking Only".
To be a customer I'm sure you agree that you need to enter the shop as a minimum.
So, enter the shop and buy something if you want to. If they haven't got what you want, or you don't see anything you fancy, or you've just gone in for five minutes to get out of the cold (in which case your status as a customer could be challenged but I doubt anyone will bother), and you cannot enter your VRM because you haven't got a receipt, see a member of staff and I'm sure they will help you out. Either that or pick up one of the many receipts that will be lying on the floor by the self-serve checkouts (some retailers provide a bin to put them in so you might be able to take one from there).
The parking conditions make no requirement to make a purchase. You have assumed that you must but you don't. Lidl's will make arrangements for you to enter your VRM if you are unable to.
Job Done!

PS - I would argue that the parking is free even if you have to make a purchase. Most Sainsbury's work on that principle. You pay to park (maximum 2 hours) and get a refund as you checkout (Minimum £5 spend in my local branch). You have to spend a fiver, but when you do the parking is free.
The sign specifies three "conditions" of parking.
It then goes on to state the penalty for breaching the "terms" of parking.
The sign does not specify any "terms" of parking.
Suficient ambiguity to argue any attempted penalty imposition for failing to comply with unspecified "terms"
The phrase "Terms & Conditions" springs to mind.
Trying to argue to a judge that you don't owe any money because you didn't breach the "terms" of parking, on the basis that the sign only mentioned "conditions" would certainly be an optimistic* strategy.
*other words may be more suitable, but may not pass the forum's strong language filter.
A fact is a fact.
The arguement wouldn't be witha judge in the first instance, it would be with the company who might just capitulate.
There is a limit to how much utter bollox I can tolerate in a pointless thread to rant about a hypothetical issue that may or may not arise. That was exceeded at least a page ago.