Ive heard that one should always contest it anyway so as to stop the clock and get circa 28 days to pay the reduced fine... or somesuch game
You can have more than that here, not that I've tested it eitherFor reference, with east Herts let you have one 'waived' fine per number plate per household. Not that I've tested it or anything
I always contest them in an 'informal representation'. I even contested when a copper pulled me over and gave me a ticket for 'Contravening Public Highway act 1898', carried a £60 fine (I gave them a cheque at the same time). I wrote them a letter asking for more details and explained the situation on how the Police officer blocked my exit causing me to go the wrong way up a one way street (it just so happened it was the local high street, was about 11pm) and back out on to the public highway. After a month I got a letter of apology and my money back 
What pissed me off once was I did have to pay one of the parking fines (think it was the 4th one in the same year) but that ticket was actually issued wrong! The street was wrong and my request for further information about the street names and where the parking areas were was met with a "We do not have that information, the ticket was issued correctly". I normally use the excuse that I was making a delivery but don't think that would of washed when I was in the VX 
I usually provide a 'invoice' proving I was delivering to one of the shops I parked near 
