I intend to discuss this matter with the Top Brass at my office. Since the UK stopped the letter it has been a nightmare, because although my annual income well exceeds the required amount, the "Monthly income falls short. This is because my UK Pension is paid monthly, but my Private Pension is paid quarterly. The office tends to ignore "average".
My thoughts are that now that I am paying Tax in Thailand, the Thai Revenue Office have verified the annual income. Immigration should (IMHO) respect that. However, I will not hold my breath.