I needed to put down a deposit on a new apartment the other day, so went to Kasikorn and used my passport to withdraw 20,000 baht OTC.

It wasn't enough so I also then went to a TMB (I think it's that) and withdrew 14,000 - this was about 1 hour later.

Checking my statement now though, the Kbank one has gone through at 46.55 to the £, but the TMB one gave 47.855!

As I was withdrawing from a Nationwide card, shouldn't the exchange rate have been the same - or does the bank convert before it sends it back to Nationwide?

I've used a Nationwide card to make withdrawals literally within a minute of each other and I get different rates although not as different as yours.

When withdrawing funds from at an ATM from an account overseas, who dictates the exchange rate? Is it the bank overseas or the bank you stuck your ATM in here?

I've never payed that close attention and just assumed the rates were set by my bank which are tied to set exchange rates.

Nationwide determine the exchange rate. ATM was almost instant, OTC may not go through for a day or 2 hence the rate and gone down in that period.