I wonder if the fitting place are sending you down the garden path here. A pick up disintegrating? I mean really?
Could it not just be that you have picked up some rubbish from a dodgy lpg station and your liquid filter has clogged up a bit, enough that the pressure over 1/4 full will push enough LPG through but under 1/4 the pressure isn't enough.
Plus, they want £600 to replace a tank? I hope they polish your knob too for that price!
A quick and rough price check on Tinley .. all prices approximate as I'm not totally sure of the exact specs ... and the bits would have to be BRC Sequent compatible
4 hole 80 litre tank, including valves/box £202 + VAT = £242.40
Fuel level sender(if BRC compatible) around £18 + VAT = £21.60
Vapourisor Pressure Sensor(if BRC compatible) around £35 +VAT = £42.00
I'm guessing there will be some pipes/connectors that can't be re-used so probably another £25 + VAT = £30.00
so I make the parts around £336 (and I might have missed something ??), which means they're charging £300 labour to disconnect it all, remove the tank, fit the tank, connect it all up, test and warranty the work for a year. They said it would take a day to do .. so 6 hours ?? .. that's £50 an hour including VAT which is about the going rate AFAIK.
I'm not so sure I'm being bent over TBH ...