I very much doubt that the failure to read the meter would invalidate the bill;

1) Your parents used the gas.
2) They should have been aware that the meter hadn't been read. Most meters require access to read them, but even if theirs didn't then all the bills would have been labelled as based upon an estimate, and listed what that estimate was. Your parents could have audited that figure at any time.
3) Scottish Power give instructions on meter reading on their website. I'd guess that they provide similar information periodically with the paper bills too.

That being said, Scottish Power would have been aware that it had been 3 years between readings and it is harsh to expect a customer to suddenly cough up that amount of cash. I'd hope that they'd be receptive to spreading the deficit over a few bills.