@Stuart ********
what concerns me is his comment of pension income etc.
Did not answer whether he had the 800k in a bank in Thailand at the time of application.
As we know UK does not endorse income.
So the question I will ask:
Did you have 800k in the bank for 2 months prior to application?
3 months after approval?
And never dropped below 400k?
If so you should have no problem doing it yourself, as long as it's back to 800k again 2 months before application again.
Also check your stamp to be sure it was obtained at your local office.
Otherwise will need the agent again.