* Why a modular system?
I can potentially buy it in parts thus spreading out the cost - summer bag now, intermediate bag later, joining bit (if necessary) last. I expect I'll mostly use the "summer bag" or the "intermediate bag" but if I do decide to go somewhere "chilly" I can put the two bags together if need be so I have that flexibility.
fair enough, but why a 'modular system'? Buy a summer bag (1-2 season, or perhaps a decent 3ish season), and then buy a warmer bag for winter - just use which one is appropriate. If your buying the Carpathian system, your spending almost £300 on two synthetic bags, which could get you down to minus 40. Are you ever going to need that?
However, a decent synthetic (if you have to go that way) bag at the same temp rating as the Tropic (which is a 1-2 season bag, no matter how you slice it) can be had for £80 from Alpkit, which weighs 1150g
https://www.alpkit.com/products/mountain-ghost-200 . So its £30 cheaper than the Tropen, comes in a 'subdued' grey, and is 150g lighter than the Tropen as well.
I'd work out how you sleep (hot, by the sound of it) and using the Thermarest (mines that old as well), to come up with the sort temp rating your after for a particular situation, and
then look at the market. Sleeping bags are something you want to get right the first time, if possible, and your not really getting a 'cheaper' (synthetic) bag, rather than an 'expensive down bag' - your potentially just buying a quite pricey synthetic bag. Cheap is not cheap, expensive is not expensive.
Down, thanks to the net, is a lot cheaper than it used to be, lasts longer, and packs much smaller. Yes, there is a reason we refer to synthetic bags as bulky - they are! Try packing a synthetic and down sleeping bag with the same temp rating - the down will be
much smaller.
True, you can't really wash a down bag at home, but you dont really need to do that all that often, especially if you use a liner. And I wouldn't want to wash a big synthetic bag much either - they are devil to dry.
Alpkit will do a decent 4 season down bag (the Skyehigh 900
https://www.alpkit.com/products/skyehigh-900) which gets you down to minus 13 (so almost the same as the Carinthia Defence 4, and warmer than the issue Arctic bag) for £200 (so about £50 more than the Carinthia 4, which is the same price as the MH Lamina Z Torch, which is lighter), but its 1.45kg, whereas the Carinthia 4 is 2kg. Tundra's synthetic bag which is rated at minus 15 is 1.8kg and is £160 (and it comes in green!). And you can get a decent 3 season bag for about £175 in the likes of Cotswolds. And there are deals, sales and of course classifieds, which might turn up a bargain.
You've already got a decent mat, and a suplus bivvy bag is always a highly thought of, as Tonyuk said, so its getting the right bag to complete the system, and I totally agree that a decent 3-4 season bag might cover most of what you want.
I have to admit to being too old to want to carry any more weight than I have to, or be uncomfortable. Since the differential between synthetic and down is so much less, there is little reason not look at both, since your just buying a sleeping bag, nothing else. And if I can get a bag which keeps me warmer, and packs down lighter and smaller, for about the same amount of cash or only a bit more, over the long term, thats a better deal for me. I can't really spend a lot of money on kit, but there are good deals to be had, if you narrow down what you want, and get it at the right time.
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