Well I for one am not prepared to engage myself, through my wife, in a potentially lengthy and convoluted discussion solely in the Thai language with my local office (because no-one there speaks English), which would almost certainly prove inconclusive (and, hence, a complete waste of our time) in any event. PND91 is the form which, as far as I am concerned, I need to complete, given that my sole source of assessable income is the UK State Pension, and I see no reason why I should now be expected to get my head around the far more complicated PND90 form, given the relatively small amount by way of assessable income involved in my case, which is fully covered by TEDA's in any event. So I'm still planning to file on the basis of the PND91, and shall take it from there.
But many thanks for forwarding an extremely useful document in my case.