I have just received a backdated rebate on my yearly parking permit; as long as you can prove you need it for work they will refund the surcharge, as well as any previous charges.
This is Islington, no idea about Haringey.
I ran the numbers yesterday with Zipcar, i reckon my vehicle costs me about £50 a week with all the associated costs.
That would be less than 10 hours a week in a Zipcar and last night when i had to work it would have cost me £71.40 (and was logistically impossible by train).