Any question answered...

I know it's a subjective question, but...

Is there actually a comfort advantage to carbon seatposts?

A 3T Doric Pro can be had from CRC for £20. A carbon version is £70+.

It will be going in an aluminium frame and as it's my first time with alu I'd like to offset any potential harshness, but I'm not bothered about weight and £50 is a big difference.

It needs to be a 27.2mm post which I understand are inherently more flexible (and therefore comfier) than wider posts.

Can depend on the exact post IME. I find Thomsons to be noticeably stiffer than some other alu posts (e.g. I swapped a Nitto S65 for an Elite on a steel frame and the difference was immediately obvious). Will also depend on how much post is showing - I swapped a Thomson out for a carbon post on my alu/crabon road bike and I can't say I noticed any difference, but it has a horizontal TT and not much post showing compared to a compact road frame.

Basically, saddle and tyres (and tyre pressure) will certainly make a much greater difference, especially if the carbon post is not designed specifically with 'comfort' features (i.e., designed to bend more than a regular post).