Currently in Thailand on Non Imm O and have recently secured teaching position starting May. Will need to change visa to Non Immigrant B and school will provide supporting documentation. Do I need to leave country to do this or can I do it in Thailand at local immigration office? Thanks for your help

Why are you on a non-O?

You do not need to leave the country, you can change the reason for an extension of stay to working without any problem at immigration when you go for an extension of stay.

Came over from UK in first instance to get married and stay with new wife. at that point was open minded about staying on to work. So non 0 seemed most logical. Now approx five months later have been offered a job and need to change visa status.

If you are married to a Thai you could also extend on basis of that. Would require a monthly income of 40,000 OR 400,000 in a Thai bankaccount. Money has to be yours only. There is no need to get an extension of stay based on work in order to get a Work Permit. Allthough as a teacher there is no minimum income requirement, so based on work you don't need an income of at least 40,000 in order to get an extension of stay from immigration.

my wife is an alien too! i am not sure understand what I need to do...as I understand it the school has been advised that to sissue a work permit for me I need to present a non imm b which I don't have.

No, you currently have a non-immigrant visa. (Your visa says non-O right, not something like TR?)

That is all you need. With the paperwork you can apply for a WP and they will give you either the WP straight away or they give you a document (WP3-receipt) that indicates to immigration that you will get a WP as soon as immigration gives you an extension of stay. After you get the extension of stay from immigration you then go back to the labour office and you will then get the WP.

Note that after you have an extesnion of stay fr one year, your wife can get an extension of stay as well as your dependent.