I bought 10m of 316 flue liner + all the fittings from a place online for about £250.. Think the place was called fluestore. Only thing I had to get seperately was the register plate. That cost £50 made to measure (also online) and was very good quality galvanised with a built in collar & trapdoor.
Fitted it myself in a day but read (and complied with) all the building regs. Didnt bother getting the jobsworth from the council to sign it off. Few plumbers in the family so I’ll wait until one of them gets their HETAS.
Hardest bit was getting the pipe round the bends in the chimney as 10m weighs quite a bit and balancing on the chimney stack, at night, in a gale and with no scaffolding was a little unnerving to say the least.