The default setting has traditionally been a 6kg Calorlite and if you go abroad a Campingaz 907 for back-up.
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Calor have stopped supply of this bottle to new customers and a standard 6kg Calor is on the heavy side.
Your best option now is a 6kg Flogas. You may have problems getting a contract on one of these due to the high demand.
I would advise getting an empty 6kg Flogas bottle of the likes of e-bay, Gumtree or Facebook Marketplace. You may be able to source one of the 5kg fibreglass see through and green Flogas bottles that used to be sold as a BP Gaslight. These are shorter and dumpy but will fit if you move the gas bottle straps across.
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Colin