There's no point seeking legal advice because such advice is only necessary when there are potential grey areas. Your case is clear cut: YOU DO NOT OWE A SINGLE PENNY..
Here's an analogy:
An airline takes delivery of a new Boeing 787 airliner. Before it's officially 'signed for' it has to undergo a large number of tests but eventually the airline's head engineer is satisfied with the quality and signs the document formally taking delivery of the aircraft. The following day, before Boeing has been paid, the airline goes bust. Would you really expect the guy who signed for the aircraft to have to pay the 130 million dollar bill? If the law worked that way, nobody would ever sign for anything on behalf of their employer!
You were acting as the publican's agent, not signing on your own behalf. The debt is (or was) the publicans, not yours. The brewery should have petitioned the receiver for payment. (The chances are that they'd have either got nothing or just a tiny percentage of what was owed to them, but that's got nothing to do with you).
Tell the debt collection agency that you don't owe them anything and you're not paying them anything.
Chris