Thai Airways Royal Orchid Plus - How to choose my seat...?

I have booked LHR-BKK-SGN, SGN-BKK-MUC in C with BMI airmiles. Can I choose my seat? I have the reservation number and found where I can choose my meal, but can't see how to allocate my seat.

If it can't be done online, would the London office do this?

Go to http://www.thaiairways.com/ then click on check booking tab, You should be able to see your info and select seat.

Yep... but then I get a page entitled "Your trip reservation". There is a box in the top right with a link to "meal/seat preferences". But clicking that scrolls down to the bottom of the page where seat number is shown as "none specified". I can't see any option to select a seat.

I normally just ring up my local TG office and get them to do it for me over the phone. Has worked fine for me on paid, TG award and non-TG award (LH, SQ) bookings.