Flexible stainless steel liners are only meant to be used as a short term solution for a chimney with cracked/crumbling clay liners.
Make sure any prospective installer is registered with HETAS.
When you have chosen your stove,seek out an approved installer to carry out a smoke leak test on the chimney,if it passes then the stove can be fitted directly into the chimney using a short steel pipe and a closure plate to take the smoke/fumes into the chimney.
So no,the chimney doesn't need to be lined.