The fact that the box has been out TWICE recently to replace synchromesh cones rings alarm bells. Sudden failure like you describe is certainly unusual with a clutch, more indicative of a catastrophic failure of the actuation method or release bearing or the fingers of the cover assembly. If I were you, I would want to be present with a camera when the box was removed to photograph or video the whole procedure. While the box was out any "normal" mechanic would look at (visually) the clutch and the release bearing assembly and report back to the office. IF the clutch or release bearing was showing catastrophic signs of wear that SHOULD have been reported to you while the box was out so that you had the opportunity to authorise the repair (cost of parts) while the box was out. (CAPITALS used for emphasis). Don't let them fob you off, if you are a member of the AA or RAC their technical department my be able to assist.