It's not a fine and is not enforcable.
Ignore them, do not reply to them. Ever. They are total bull-crap.
The only thing you do not ignore is court papers, but you will win the lottery before these arrive.
Josweet - with regards your specific instance with totalparking (herein known as Total Parking Solutions - TPS) - it is not a fine. It is a PCN (Parking Charge Notice) which they can if they want to, take you to court over (they are within their rights to claim something).
However - the chance of them taking you to court over £40 is so slim that it is nigh on never going to happen. You have to ask yourself how much it would cost them to get it to court in the first place - it's not free to kick something like that off!
They can ask you to pay for it and add on some crazy 'late payment' fees etc. but again, it costs a lot of money to kick this off from their end.
Simply put I would never reply, send money or respond in any way unless court papers land on your doormat.
They are simply after those people that cave in and pay the PCN.
Zippyrude - josweet posted on the forum here for comments about whether this is enforceable. I answered. Pepipoo will simply give advice just like I am doing. Because thats all it is at the end of the day, advice.
If you really want to get serious - go get some legal advice from a motoring solicitor (whatever that will cost). But then - that defeats the whole point doesn't it...