Primaloft/down jacket for layering?

The Pattagonia Nano puff  is what I think you are looking for. You can use it as an underlayer,  much warmer than fleece,  infact much warmer than you would think it capable of.  compress to small very small size.

It is a bit over your budget, but the PHD Kappa jacket is extremely good - totally waterproof outer fabric, good Primaloft insulation and also a very practical jacket style cut.

I tested and reviewed one of the prototypes in Lapland back in February and it was good to -20'C with fleece and base layer under it, when I was active.  I think this report is still listed on this site somewhere...?  Since then, any wet/cold UK weather and it has been my jacket of choice.

James

Generator vs 2 microfleeces on top? Still warmer. Much more so than one.

Big difference with the generator vs nano puff is the weight difference - 100g/m2 vs 60g/m2. Even 60g/m2 is a lot more than one microfleece mind. I'd guess sort of similar to two.

If you want a little more than 1 microfleece and very light then Motane's fireball say, or the OMM rotor smock or that TNF one etc.

Oh I really would consider the idea of sizing it to go over your shell. Primaloft can take that sort of treatment/saturation OK, and the convenience gain when you're climbing in your waterproof is obviously very real.

If you're worried about wet weather, don't bother with down; it won't be worth the hassle.

I've grumbled elsewhere about using synthetic insulation as a mid, so I won't repeat myself. My suggestion is to keep one microfleece and carry something warm like the generator for stops; size it to fit over windshells or softshells or whatever you usually wear on the outside (assuming you aren't of the 'goretex all day, everyday' school of dressing). Chances are that a non-lightweight hardshell will fit okay over the top of it already. Overlayering is the way to go.

A waterproof warm layer like the kappa would probably be a bit inconvenient to carry around on the offchance you'll need it, and its probably a bit warm for active outdoors use in the UK. Blah blah blah Paramo blah

Chris89 wrote (see)

I'm really just looking for a mid layer to warm me up on a rest stop.

Synthetic filled items are at their best when used as an outer layer. A basic, minimally featured bag, basically. Throw it on over everything else you're wearing. If the weather's pish, why would you want to remove outer layers to add a warmer midlayer?

Thats the basic point of synth insulation It holds its warmth OK when saturated then dries out afterwards. So any set up with it will work reasonably.

You can get more resistant fabrics than the nano puff without going to the extra weight/expense of a built in shell mind. Just coated stuff as per pertex endurance - large parts of ME Fitzroy/RAB generator alpine/PHDs sigma etc. Not fully waterproof but quite close and a fair bit cheaper/lighter than the fully insulated shells.

Also there is the Rab Photon jacket -

a little cheaper than the Generator alpine.

I've had one for 3 years* I mainly use mine as an overlayer, but if it's really cold n wet, in Scotland once up on a windy Munro ridge walk, I have used it as an  midlayer under my eVent shell and it was fine. Mine is Black, and I am happy to wear it casually too.

*It's Primaloft sport now not PL1 , but I wouldn't get too hung up on the difference myself - PL sport will probably retain more loft for longer though it's slightly less warm for it's weight than PL1 and  a little less compressible - not  that that I compress mine much as it's not a good thing in the long run.

my 2p worth....a windproof would also help.

a jacket is heavier and perhaps not as good a fit, but you want something to throw on top [and take off] quickly .

Rab photon is great, but maybe not warm enough? easy to fit under shells as well if needs be.
Snugpak and Keela are certainly worth looking at. I'd always go for a hood, but make sure you can stow it to stop it flapping and filling with snow