You'll need to get it swept anyway, so ask the sweep to do a smoke test and to give you a view on whether it would need to be lined.
If the draw is OK, and the chimney is not leaky, then no need, as long as you're getting it swept quite regularly.
But you'll still need to get it signed off by building control to make sure your buildings insurance covers you in the event of a fire. And I'd advise getting them in to witness a successful smoke test rather than them coming in after you've got the thing fitted with register plate etc and then demanding you line it.
If you're not lining, DIY it. Read part J of the building regs to see the requirements (hearth size, thickness, proximity to combustible materials etc). If you go through a HETAS installer you will pay through the nose.
Even if it needs lining, you can do most of it yourself and get your sweep or a roofer to help you drop the liner and fit the cowl.