I'm planning to work in Thailand for a UK company that does not have a presence in Thailand (i.e. no office there). I will be paid into a UK bank account and will then withdraw that money from an ATM as and when required.

As I understand it, I should pay Thai income tax rather than UK income tax because I will be living in Thailand for more than 180 days of the year.

I have two questions regarding my situation and would really appreciate some advice:

1) Am I correct in understanding that I should apply for a Non-Immigrant Type B visa but that I do not need a work permit.

2) I understand that one only pays Thai income tax on money brought into the country (i.e. money withdrawn from an ATM in my case). If the money in my bank account comes from a variety of sources (e.g. salary, dividends, interest etc) then how can I, in practice, say which of the sources of income a particular withdrawal relates to when it comes to filling in my Thai tax return.

Thanks for any help.

1) Am I correct in understanding that I should apply for a Non-Immigrant Type B visa but that I do not need a work permit.

You require a work permit to legally perform work in Thailand.

As you state you'll be here for more than 180 days, then this is not a temporary "business trip" from the UK office.

You will need a company in thailand to give you an invite letter for you to apply for a Non Immigrant B visa at a Thai embassy in UK. You can then enter thailand and get an initial 90 day permit of stay stamp at immigration, get your work permit, tax id, pay PIT(personal income tax) on your salary, and then get immigration permit of stay extensions on the basis of having a work permit.

As your UK company has no office in Thailand, you might find complying legally to be rather tricky.

2) I understand that one only pays Thai income tax on money brought into the country (i.e. money withdrawn from an ATM in my case). If the money in my bank account comes from a variety of sources (e.g. salary, dividends, interest etc) then how can I, in practice, say which of the sources of income a particular withdrawal relates to when it comes to filling in my Thai tax return.

kiakaha, has cover most of your questions but to clarify a couple of points:

Withdrawing money from an ATM doesnt attract income tax in Thailand

The Thai tax man will not be really interested in your external sources, the premise being that you will have been already taxed at source, whether true or not..

Chances are your UK company will have to tax you in the UK, and the income tax laws the way they are in the UK, you will not get around this with a UK based entity.

As Kiakaha has said you cannot work in Thailand without a WP and this needs to come through a Thai entity and you pay tax here, so you dont have too many options legally in Thailand. To be fully legal you need to be taken on by a Thai company, which for the purposes of your tax, could be interesting for the UK company as the Thai company can invoice the UK company etc etc...