In reply to Greenbanks:
I would first contact the hire car company as I presume they must have provided the parking authority with your name and address to see if they can shed any light on the matter, specifically to check your concern re. authenticity.
In my experience, ignore it at your peril. I omitted to pay a single congestion charge whilst travelling through Oslo in a hire car a couple of years ago. They traced me via the hire car company and started bombarding me with letters in Swedish/Norwegian, all of which I ignored. Eventually, I received a letter from a UK debt collection agency who'd been sold the debt by the Swedish company who operates Oslo's congestion charge. A payment of £300 was required within 15 days otherwise they'd take me to court etc. This amount was the original penalty, plus penalty charges for ignoring all letters/ time limits for payment plus the debt collection agency's 'fee'.
The original amount owing was the equivalent of £10.13!
Clearly we no longer live on an insular little island where Johnny Foreigner can simply be ignored. IMHO, the possible consequences of not paying just aren't worth it.
Food for thought if nothing else ...
Dave