Why did you park outside the lines though?
If you don't adhere to the parking conditions then you can expect to be fined. I always thought it was a "rule" that you had to be inside the lines.
That said, I think the best course of action is ringing the
halfords store, tell them you got a fine (don't say exactly why) when you were in their store and you have a receipt to prove you were shopping, and they will make the fine go away. I've done it several times for customers. And for myself as it happens, Mr6 picked me up one evening after work and dropped me off again the following day. Parking eye decided the car had been parked there all night. Feckwits.
Slightly ot, but a friend recently had a huge rant on facebook about being given a parking ticket whilst at the park with his kids. He had parked, not paid for parking despite knowing it was a pay and display car park, and subsequently got fined. In his mind this justified calling the parking warden a "effing geriatric paedophile" - in front of his kids! Apparently it's quite unfair to be ticketed in such a way when you're just letting your kids play in the park.
I'm not sure why people get so agitated. If you don't want a ticket , don't park "illegally" Much like if you don't want a speeding ticket, don't speed.
We parked outside the bays at the metrocentre once, got a fine, so did everyone else who'd done it. We paid. Learned our lesson and didn't do it again.