If the parking place was free (ie - no charge was made to use it) then, having a ticket or not having a ticket makes no difference when it comes to the monitary consideration regarding parking. displaying a ticket (or failing to do so), would appear to make no difference (you cannot be compelled to produce a receipt for nothing). If there were a time limit, then, naturally, this would not apply as your "free" use was time limited and the argument that the ticket was required to ascertain the time would appear to be valid. If, however, there was no limit then, logic dictates that at ticket was superfluous and was only required in order to impose charges (which makes the free parking idea a nonsense). It would be tantamount to an invite to park, merely so they could impose charges - which is surely some form of entrapment. If the latter is the case and the only notice regarding this is on the ticket machine, then (as you would have no logical reason to go to the machine) - then the "fine" (AKA - the invoice - or request to pay) is invalid.
If there were clear signs (take some photos if there are not) then, ignoring them may well mean that you have to pay the invoice as (as far as I know), ignoring them would mean that you accepted (but ignored) their implied contract - but accepted it - by parking there.
If that makes any sense - could someone explain it to me?