Have I got this right?

[2lz2p]

I plan to spend 30 days in Pattaya in December/ January. On arrival, I will receive a 30-day visa. If I decide to make a short visit to Cambodia by air before my 30-day visa expires, do I receive a new 30-day visa on arrival when I fly back into Thailand, thus allowing me to stay longer in Thailand than 30 days?

Yes! As mentioned by others, you will be entering "Visa Exempt" which grants you a 30 day permission to stay. If you leave Thailand and return by air, you will received Visa Exempt entry for another 30 day permission to stay. There is no limit doing this by air, but if Visa Exempt land border entries are limited to two per year. However, if done by air several times, especially back to back, Immigration may question whether you are really a tourist and in most cases tell you next time to get Visa, but depending on the Officer, they could deny entry.

On the other hand, I understand that I can obtain a 60-day visa from the Thai Embassy in London but there is a fee and some paperwork to complete.

If you obtain a Tourist Visa, when you enter you will receive a 60 day permission to stay. A Tourist Visa is valid for entry from the date of issue up to 90 days later.

Finally, if I enter on the standard 30-day visa on arrival is it possible to obtain a short extension - say by a further two weeks - at the immigration office in Pattaya for a fee?

Both a Visa Exempt (30 day stay) and Tourist Visa (60 day stay) can be extended at any Thai Immigration Office in the country for another 30 days - the fee is 1,900 baht.

As mentioned by another poster, your airline may deny boarding if you do not have an onward ticket within 30 days of your arrival if you do not have a Visa (visa exempt entry). This is because it is an Immigration requirement which could make the airline liable if denied entry (even though that is very unlikely as Immigration Officers at the airports rarely if ever ask to see an onward ticket).