@Duncanc *********
i was in exactly the same position last year and opted to pay the fine at the airport because it would save the hassle of a day at immigration to get the extension.
I'm back in Thailand now but if I had that time again I'd probably get the damned extension. Every time I go near an immigration officer now I get the warning about overstaying. Nothing too dramatic, but it does have an effect on the proceedings later on. It just makes you a suspect to overstay again and it is a dark mark on your passport if you're a regular visitor.