Here is an experiment, outside, have a fire in the garden and place a sealed tin of beans on it. Puncture a tin of beans and place that on at the same time. Stand well back and wait as they heat up.
If it is sealed. or can become sealed in use then I believe it becomes a pressure vessel and subject to all sorts of other regulations.
Does capped mean sealed though, or just protected so that hot air expanding out of it isn't directed to a person standing nearby?
Legal requirement for a liner? Not such a thing (see below), unless it forms a part of a sales contract with you and a stove supplier. Nearly all manufacturers of anything will recommend stuff but not demand it (legal issues on both sides, if they tell you to spend money doing something and turns out you don't need to, you can claim on them you see). Recommended though, but if you chimney is in good condition then you will be OK. What is the chimney at the moment? If it is lined you might get away with just a stove install.
I'd be tempted to take it out at a suitable time and get it replaced. Removal could be a DIY job - the boards are - but the stove will be heavy and need a couple of you to get it out the house. Get the sweep back though - I'd recommend that for a first time sweep anyway - tell them the stove is being replaced and you want the chimney swept and inspected (they will more than likely put a smoke pellet in, check all the way up for leaks, check the chimney pots and chimney). Do that in advance of the stove being installed - just in case you need remedial works doing, be a shame to watch a brand new stove doing nothing while you wait for a builder to do works.
If you have a chimney liner just now and if the sweep says it is good to go then I'd get the installers to remove the old stove... they will know how to do that without damaging the liner if you tell them to reuse it.
Final couple of thoughts, does it work OK as it is? and from above, sweeping it yourself, can be a DIY job.
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