Calum ..
You must turn the bottle valve on SLOWLY...
Opening the valve quickly can cause the pressure regulator to block with dry ice and thus stick open.( As suggested further up this thread )
When this happens your L.P. relief valve will open but cannot cope with the FLOW of gas created by unregulated co2 ...thus the pressure rises very quickly to that of the main bottle...and something goes BANG!

The L.P. Valve should really be sized to accomodate this scenario ... but I don't belief that m/any people do.