Hello,
I would stop paying the "informal" rent now, until you start getting some real traction from the property manager and I don't just mean one quick call with some bull.... excuse for ignoring you this past number of weeks etc.
Have you historically been getting receipts for payment of the "informal" rents being paid btw ?
Next stop, is the Receivers. I expect you know who they are. Write to them (by registered letter) indicating that you are the sitting tenant and want an immediate meeting, stating that the property maanger has not been engaging, returning calls etc. Your ultimate goal here is to secure a lease, assuming you are not in a position to make an offer to buy the property - which you may be, if you have built up decent trading figures for your business over the 3-years to date etc.