Hi Craig,
Basically a "conventional" gas bottle is made from low-strength steel and needs to be quite thick to withstand the maximum design pressure - and that makes the empty bottle quite heavy.
Companies like BP introduced a plastic composite gas bottle (across most of northern Europe) a few years ago - and the empty weight of these is around half that of a conventional bottle. Likewise it's been possible in Holland to buy aluminium gas bottles - again saving considerable weight.
In the UK, Calor have introduced a higher strength (thin wall) steel bottle - that is dimensionally the same as their old bottles - but like the BP bottle, weighs around half that of the old bottle.
They should be readily available from Calor outlets - and since most CC camp-site wardens seem to have an arrangement with Calor - you should be able to source them whilst on site.
Robert