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The Alpkit Hunka is a great bit of kit for the price, but the lack of cover for the face end is always going to limit the usefulness. The problem is that bivis are a bit specialist, so a good one is going to run you near the price of a decent tent anyway. I have a fairly basic Rab bivi [edit - Rab Storm, same as you just picked up] but the head end zips closed (you shouldn't close it entirely when in it) so that helps keep the weather off. Some of their spendier bivis are a bit more clever at the head end with lashing points and so on so that you can get a bit of space around your head - really important for minimising condensation - but in my opinion once you start adding hoops etc. to the design, I start to wonder whether a tent isn't a better choice.
The bivi itself is never going to contribute much to the insulation; its job is to keep you dry. What sleeping bag are you using and what temperature rating is it? You probably want to go heavier, probably synthetic for bivi-ing (because moisture build-up can be a problem and moisture kills down insulation) although that will add to the bulk. You might also consider a sleeping bag liner as this can make a big difference to warmth - Cotton and silk are traditional, but Thermolite fabric is synthetic and the fibres are hollow, making it an excellent insulator. Also, ditch the foam mat and get a self-inflating Thermarest-type mat; they're warmer and pack down smaller.
General_Strike-
I don't want any hoops so RAB Storm and Rab Ascent are kinda where I'm at. I don't use a sleeping bag, well I haven't yet, but it's likely I will if I'm riding in winter. I only have 2 down bags and a down jacket and don't fancy buying a synth one so I'll just be careful leaving a vent gap in the bag. I have a silk liner but don't usually take it cycling (used it touring years ago). I've got an old thermarest I might use for audax etc. in the UK but I'd still like one of those NeoAir mats. I went with a foam mat because some people I spoke to complained about p_nctures and not being arsed to inflate while racing.
hippy
Bivvying in UK winter.
I get too cold in an Alpkit Hunka with a down jacket and a foam mat and thick layer of body fat so what should I try? Is there a better (read: warmer) bivvy option that doesn't add much weight/pack space?
I will also need a solution for TABR/TransAm where I might be sleepy in/near the high mountains.
Didn't want to pollute the tent thread any more...