I have just been to fill with gas again after letting it run out and even try to use the dregs. The nozzle when attached started hissing, is there not a valve at the tank end as well as the filler? I removed the nozzle and remade the connection with no benefit. Started the pump, which gave a few cc of LPG before shutting down its power! Got that restarted and the pump shut down at 29 litres. I stood there for ages to get a little more in and got bored at 30.332 litres. Fuel gauge shows just over halfway, although I appreciate that they can't be relied on.
So, could I have a faulty unit on the tank (Tinley says not a chance) or is there somewhere else I should be looking?
There should be a one way valve at each end of the filler as you suggest - one to allow flow in through the filler and cut it off when you disconnect, the other at the tank to allow flow into the tank and stop it coming back out.
The hissing you mention could be one of two things - either the tank is really empty and the hose is allowing the pressure to equalize, or its leaking, The former is nothing to worry about (does it happen with the tank not empty?) the later should be obvious (you'd notice a smell of gas as your filling or possibly be able to see it either as a haze around the filler, or dripping out the bottom edge). this could be down to the pump rather than the filler you have though.
The behaveour on filling does happen sometimes where its slow to start, or starts then stalls before getting going, usually just holding the button is enough to get it going after a few seconds.
It doesn't take a lot of alteration to the alignment to make a noticeable difference to the fill level though - I've seen a 10 litre difference on a 50 ish litre filling tank just down to sloped ground near the pump when filling. The colder weather usually ends up with more going in though so yours still sounds quite low all things considered, so something appears to be wrong.
Tinley tech are usually quite good - I'd assume you have the right valve fitted to the tank but may be worth checking the valve itself if you can see the markings in anyway?