I have just had the O visa put in my passport with date from 15th December to March 14th. Unfortunately due to health tests, I may only be able to travel until the end of January. Does the three months start from my arrival in Thailand, or, from when you applied for the Visa? If it is from the date of application, does that mean whenever I arrive, I only have until March 14th?

There is an 'expiry date' on the visa.
As long as you arrive before that date then you will be granted a 90 day stay in Thailand from the date you arrive at the Immigration Officer's desk!!

Additional
Remember:
To enter the Visa number in the space provided on the TM6 arrival form.

Check, before you leave the Officers' desk, that you have been given a stay of 90 days.
It is quite possible that the IO could use his 'wrong' date stamp & incorrectly give you only 30 days. This would be immediately corrected if you point it out while you are still standing infront of him, it is a rather longer process if you need to correct it later..

Thanks Bob! Thailand doesn’t make things at all clear regarding this question do they? London Embassy never have time, Hull consulate seems to put in black and white it is valid from visa application not entering Thailand. I thought it was from entering Thailand it started but if I got it wrong, it would given me many problems!

Remember also, if you happen to leave Thailand at any time during the 3 months validity of the visa (e.g. a day trip to Vientiane), whatever the time period that remained of the visa is lost.

You will only be stamped back into Thailand for the length of time according to the rules pertaining at the time at the border point of entry.

Thanks Jimbo, I will bear that in mind.