Hello, I run a small Charity Community Centre and we had a room booking from the NHS for every Friday for 3 months. After I issued the invoice they raised a P.O and they paid. They then emailed me the next day asking if they could book the room for another 3 months. Which they did. I then issued them a invoice on their last session which was 50 days ago and I have had no reply from the person who I was dealing with in regards to making the booking. I have tried contacting the Accounts department but they say they will not pay without a Purchase order. The only person who can raise the Purchase order is the person who is ignoring my emails. Where Can I go from here?