Going to apply for one of these from London Embassy. Question is do I need to leave every 90 days or just report to local immigration? Also, can you get 12 months or 15 out of one? Cheers

If the visa is issued just before you leave for Thailand, you can get almost 15 months out of it. Each entry is for 90 days, and you can get a new entry immediately prior to the one-year anniversary of the visa being issued. You do not need to do 90-day reports. Normally, you leave and reenter Thailand every 90 days. If you are married to a Thai, or have a Thai child, you have the option, once per entry, of a 60-day extension to visit with them (which also means you may be able to get nearly 17 months out of your one-year visa).

4 hours ago, wildwildbill said:

Question is do I need to leave every 90 days or just report to local immigration?

The visa only allows a 90 day entry so you must leave at the end of those 90 days unless you qualify for an extension.

You only do a 90 day report if you have a visa or extension that allows you to stay in the country longer than 90 days.