My flight out on 28th April has been cancelled and I have been put onto a flight on the 29th at 9 am instead.

I have been here for 2 months on a tourist visa and the exit date in my passport is 28th April.

From other posts I assume I won't be fined but will I get an overstay stamp?

Thanks

Correct but even if you get the stamp it does not mean anything so nothing to be concerned with on a short period such as that.

But if concerned you could easily extend your stay 30 days for 1,900 baht at Immigration on Monday or Tuesday (if you believe stay may be longer it might be good idea). But if they see you are leaving only one day late they will likely just say forget it.

Your airline might be able to help, if the change was beyond your control

Cheers.

I'm going to try and get some kind of proof that it's not my fault at the check-in desk but I doubt I'll get anything.

No worries, but I would prefer not to have an overstay stamp in case things change in the future regarding these things.

I'll post when i'm back the UK for future reference.

Back in the Uk.

The airline were no help and didn't seem interested, maybe they knew it wasn't a problem.

At immigration I smiled and told them up front that I was 1 day over because my flight had been cancelled the previous day.

The officer checked my passport, smiled and said no poblem sir, enjoy your flight.

In short, no fine and no overstay stamp.

I once got an overstay stamp for going 15 minites past midnight. When I pointed out that I had been stuck in the immigration queue for 45 minutes I at least got an 'unlucky' from the officer. He also said no problem in the future.