Apologies for not posting sooner.
It turns out that the bulbs were correct, but the connection wasn't great. Checked out the price of new bulb tray at TPS who wanted £16 so I messed around a little and managed to get a good connection. So far so good. No disco and no loss of power.
So one to watch in future I guess. Amazing that a dodgy connection can cause total loss of power when lights are turned on.
Thanks for the
help and suggestions.