I've got it into my head that the door frames in our (work-in-progress) flat look terrible. They are the plainest design imaginable, lots of chipped edges and layers of gloss paint.

Is it worth replacing them to increase the flat's value? Is it even possible to replace these at a reasonable cost?

There's such a thing as a door lining kit but maybe that's for new build only... Advice greatly appreciated!

I have no idea how much they would cost to replace, but I was just wondering if it would be cheaper and perhaps easier to try and renovate them? I don't think most people would notice the design of a door frame when looking around a house, and my opinion would be that this isn't an area where much value could be added. Of course you'd want the frames to look as good as possible, but perhaps if the paint was stripped off and they were either repainted nicely or varnished then they might look better. You can also, I think, get wood polyfilla to deal with any holes or cracks in the frames.

May I suggest you try stripping some of the paint off first. You may well find that underneath it all, there's very little wrong and a bit of elbow grease , filler and sander might do the trick. You should also be aware that when you say "door frame", it is actually made up of at least 4 different pieces of wood each side, and the top. ( main frame, door stop, architrave each side.) The last three can be removed and / or replaced easily, providing they aren't intricate mouldings. However, you can now get a huge variety of mouldings, which would make the frames as good as new.
You'll find replacing the linings much cheaper than replacing the whole frame, as that would involve re-plastering as well, and a lot more mess.

Door lining kits are very cheap to buy around the £12 mark Howdens,Wickes etc. and Architrave surround mouldings are cheap enough. Go for a decorative moulding im MDF like Torus or similar, a good chippy can bang these door linings into place in no time with a good chop-saw, nothing worse than old badly painted and chipped woodwork with a brand new door fitted and don't forget your going to be the one sitting there thinking 'I wish I had replaced-----'

Thanks for the advice. I had a go at stripping the paint last night with a heat gun. After 1 hour, I'd only done a small patch and was then running around furiously trying to switch off the smoke alarm because of the smoke I generated! So I'll check out those door lining kits!

Will it add value? – It depends on the aims of your project, but if the house has a ‘period’ charm, then I think it’s worth making sure that style of the skirting, architrave, dados, picture rails, and plaster cornicing, etc., are all ‘appropriate’ for the house. It’s the same sort of thing as getting the windows ‘right’ – very easy to ignore but does make a difference in ‘pulling everything together’. But if the period thing is not important, then I would go with Melboy's advice on getting clean sharp lines.

We've tried to renovate all the doors and frames. I've kept all of the original doors and linings (yes, a lot of filling and sanding) but I have had to replace some of the architraves.

We went to a good timber merchant and we got a match so you can't tell which are renovated and which are new.