ok sorry i though you wanted flue rather than for part of the heater,
you can get cheaper twin wall flue fromt the plumbers merchants its non insulated and made of galv steel rather than the expensive insulated stainless stuff.
you will also get a 5" to copex adaptor for the end of it, copex being the trade name for your flexi pipe if its proper chimney liner.
i will have a rake in my lockup if i get a chance i may have some bits,
you also get flat clamping plates for the liner as well
also on ebay in stirling 290675497854