Sorry for my ignorance. An ISO 9000 assessing company in the UK has asked if I would carry out assessments in LOS on their behalf. It is apparently something requiring a native English speaker (I don't know the full details), although this is a new contract. It is more beneficial to use me as I am often in LOS anyway, rather than fly someone out from the UK or Aus.

My question is: if I am undertaking a consultancy for a UK firm which has a contract with a Thai company and payment being made in GBP by money transfer to the UK, taxable in the UK, what sort of work permit would I require?

You need a work permit:

Under the Act, the term 'work' is defined very broadly. It includes all activities, which entail exerting one's physical energy or employing one's knowledge, whether or not for wages or other benefits. Consequentially, even foreign volunteers are normally required to obtain a work permit even though the alien will not receive any remuneration for services performed. If the activity of an alien performed in Thailand results in income or benefit to another person, a work permit must be obtained or the foreign citizen will be punishable under Thai labor law.

http://www.ethailand.com/index.php?id=1820

You need a work permit:

Under the Act, the term 'work' is defined very broadly. It includes all activities, which entail exerting one's physical energy or employing one's knowledge, whether or not for wages or other benefits. Consequentially, even foreign volunteers are normally required to obtain a work permit even though the alien will not receive any remuneration for services performed. If the activity of an alien performed in Thailand results in income or benefit to another person, a work permit must be obtained or the foreign citizen will be punishable under Thai labor law.

http://www.ethailand.com/index.php?id=1820

Many thanks for your help.