The Tarntawan is under new management so maybe things have changed. Also, I believe Hotels are not allowed to undercut booking sites. Maybe this is why you haven't heard.
I think you are in the UK and, if so, maybe you could do the same as I do which is to book through Expedia.co.uk I have used them many times and their rates are always the best I can find. It also has has the advantage of paying immediately in GBP from your UK bank account.