Chris Hogg wrote:

The problem is that I have a pottery kiln that weighs 110kg (the 250kg
figure

Assuming you don't have a gantry above it, then I'd go for jacking from
beneath. That weight is more than most roof timbers would appreciate.

I'd probably use a load of small jackings and repeated stacking with
wooden packer blocks. OTOH I have plenty of timber to hand (there are
always pallets though). How many trolley jacks can you borrow ?