Good evening ,so I need to go to Samui Imm before the 11th of may 2013,extension Imm O based on marriage to Thai National,it will be my 4th year in a row,but the thing is i want to go back to the U,K .on may 20 th for 6 months.What my best options. Thank you for all reply's.

You should have done the one month stamp earlier and collected the rest before your trip then got a re entry permit, to last you till may next year.

You still can try to do that before the 20th.

If you go on the first day after Songkran with all of the correct documentation you will have to come back in 30 days after the "under consideration' period and collect your extension, and buy a reentry permit. but it is doubtful that it will actually be there before the 20th of May especially since you are on Samui and it's just after Songkran

The best option is to go to immigration and request a 60 day extension because you have a Thai family. Go home. Then apply at home for a single entry Non-O just orior to returnig to Thailand and extend that for marriage purposes during the last 30 days of your permission to stay

Per above, you only have 1 option. Get a 6 day extension and apply for a new non-O visa while abroad.