We intend to move back to Thailand in 3 years. In the meantime I am considering using the 3 month o-visa. If an application is successful is there a period of time within which I must use it. Say I applied, but can't get away for a couple of months, is it still valid and, if so, would I still get 3 months use?. Thank you 

Yes and yes.

 

Yes, yes and yes.  From the UK embassy website;

• Single Entry Visa will be valid for 3 months from the date of issue. You must enter Thailand within the validity of visa.  However, the permitted length of stay is up to 90 days from the date of your arrival in Thailand. Please carefully note the immigration stamp in your passport to make sure that you do not overstay your visa.