I'm after a new mid-layer jacket that works well on it's own throughout the year (except maybe the hottest summer days) or underneath a windproof/waterproof on the really cold days.

I generally wear a thermal base-layer as well so it will go on top of this.

Any recommendations?

Sounds like you want some sort of softshell to me, look at patagonia, montane, mountain equipment etc

montane dynamo on offer at cotswold on line or often available reduced on a google search. designed to cycle or fell walk in not cycling specific, smart enough to wear around town as well.

the endura mt500 jsersy jacket looks like it may fit the bill as well

Anyone used the Gore Bike Wear Alp X III Thermo?

No other suggestions?

Won't any old bike jersey fit the bill? Not much point in a windproof mid-layer IMO. If it is cold enough for a windproof then you will be wearing your windproof (you imply you have one), if not a normal jersey would do, no?

Also, it has to be pretty damn cold to need more than a base layer under a top quality windproof IME. I find my Gore Oxygen SO good to around zero and have never worn it with more than a Helly underneath. But then I don't ride in minus 10.

But I don't have a top quality windproof/waterproof: It is a rather tatty Endura that is fine as a windproof/waterproof but provides little thermal assistance. Plus it has no lining and is therefore fairly uncomfortable next to the skin.

As I said, I am looking for a new jacket that will also be worn on it's own during the autumn/colder summer days, but also to be used with the waterproof in the depths of winter.

Well, perhaps look at ditching the Endura and go for a soft shell as mentioned?
I first thought that a soft shell was not what you were after as they aren't really mid layers, but if your outer shell is terrible then look to replace that.
I either wear a wind proof Gore N2S outer shell, or a full rain proof with a base layer and maybe an extra (thin) jersey as a mid layer if it's freezing. If I wear anything with any warmth under a shell, plus a warm base layer, I would be warm enough for walking, never mind cycling! And I'm skinny and feel the cold!

i have a selection of santini road kit and one jersey in particular that does what you are asking for.

its like a normal long sleeve jersey but thinker, windproof (ish) and high collared. normally wear it on its own, if it chucks it down ill throw on a windproof (likewise if i start to get cold)

try looking at roady kit, from my experience there is a much better range of thicknesses, windproofs etc.

think i've just made it obsolete actually with a new jacket that is so spiffing i can wear it with just a base layer. windproof, waterproof and also not sweaty. true black magic.

Agree with above – my "Gore Oxygen SO" is so versatile that half my kit is now no longer needed/used. It, plus a base layer, is all I need pretty much all year round (windproof on its own for cool days, plus base layer on very cold days, base layer on own for warm days). It is even fine in all but torrential rain (it beads brilliantly) – in fact I haven't used my eVent jacket since having the windproof.

Get a softshell jacket, then you can wear a merino baselayer if its chuffing freezing, or a short sleeve jersey if its just a bit too parky to go out in shirt sleeves.

I have an arcteryx epsilon, its awesome biking, invaluble hillwalking and smart enough to wear casually.

Arcteryx kit is lovely 🙂

pearl izumi or gore here