Offshore -- Onshore ? Convergence -- What Happened?

[mgsrus]

As the GBP rate on the rise? thread has followed the usual pattern of disappearing up its own pipeline, I will [naively?] post my related question as a new thread. Apologies to all if it goes the same way!

Most of us who are based in GBP, don't understand how forex works and why it performs the way it does ---- likewise I believe many who think they do ---- all we are concerned about is the number of Baht that end up in our pocket when we need to exchange our hard-earned [for me anyway] spondulies into this walk-on-water currency that enables us to build our house, give the missus her 'salary', buy beer etc etc

The onshore rate for GBP/Baht [amonst others no doubt] still varies significantly to the offshore rate -- this morning SCB opened with 54.155 and EX.com gave the rate at 54.6352. They both jump around more than Zebedee and that was the XE rate 2mins before I checked SCB, 2 mins after XE was lower at 54.1314 -- which confirms convergence at least -- but why the difference? There was a lot of chat last year about the rates being allowed to converge under 'market forces', which they did many times, and thereon being locked into one rate -- was I dreaming? Did 'somebody' change their mind? Did the financial crisis put paid to that, if so why?