Connecting gas cylinder to fill point in France

[Paddington]

The more I ask...the more confused I get....
I have a 15 litre undercarriage lpg tank. If I stay at an aire in France over winter, I won't have enough gas to heat the van. I want to avoid having to go down to the valley mid week to top up the tank so...
I have a French adapter to connect the French filler to my tank and I have a pigtail so....can I buy a gaslow refillable cylinder (not fitted to the van) and whip it out at the petrol station and fill it when filling the lpg tank? Is this legal? What else do I need?
I'm not technical so please explain in lpg for dummies speak

 

[Shaun Witts]

I don’t know about France, but most LPG pumps in UK have warnings to not fill portable cylinders. I often think that the cashier takes a careful look before engaging the pump for me.

 

[Paddington]

Oh. Didn't know that.