It sounds like your fillstop is sticking.
Hitting the tank with a soft mallet will probably free it up to allow you to fill but it is likely it will stick again. I'm not sure the fillstop can be repaired, it might be a replacement tank. Ideally you need to visit a LPG specialist with experience of your system.
Unless your tank is full, which you don't make clear (LPG gauges are notoriously inaccurate), and the LPG system is faulty?
You would have a far better response if you had posted this in the 'Having Problems' section rather than 'Alternatives to LPG'
To help identify your Vauxhall LPG model, go HERE