Mat,
you should have this fixed by now, but I would just drain the tank and purge the fuel line, and wouldn't worry about changing over the filter.
You can put some petrol in with your diesel and it won't damage the engine ( All petrol will cause it not to run though )
You shouldn't have damaged your motor in any way. The only real damage I think you can do, is to keep cranking and cranking to get it started as this may damage the Fuel Pump through lack of lubrication

