Sorry to hear you guy's are getting these demands.
Not a uk tax expert but in a number of jurisdictions I have dealings with if your employer cocks up the equivalent of PAYE it is THEIR problem to resolve, they pay the tax and then can ask for it back. Almost unenforceable as they were negligent.
Maybe it is worth checking with a uk tax expert, some do free short consultations to see if it is worthwhile getting them to do things for you.
Good luck guys
Come to think of it as a beancounter (for my sins) if there are a load of you guys getting these demands I could contact one of my firms UK tax guys in the UK and ask if they can help.
They could then do you a group rate for dealing with it at miniml cost each! mail me if you are interested.