Just confirmed I have the same tank and must be either a lock somewhere whilst I filled it or a faulty fuel sender. Probably the only way I am going to confirm it is measure the amount of fuel in it and refill it. The sender will be replaced to eliminate it all together.

Thank you so very much for your input. I had the LPG tank emptied to replace the sender and took 112 litres to refill it. I had driven 450km and was just on half full when the workshop emptied it. Not bad considering I drove it pretty hard coming home from picking it up.