lightweight sleeping bag

I have and like a lot the MH Phantom 32.  Cotswolds have it for £180.  Pretty warm 3 season for 640g.

They also have the Phantom 45 for £159 at 480g. How they reckon it has a lower limit of 0 degrees is a bit dubious though as 45 equates to 7 degrees C and the 32 which equates to zero is usually rated slightly cooler.  Good quality tiny packing bags anyway.

Alpkit PD400 at £150 is meant to be good but at 750g is a bit heavier and bulkier though warmer.

If you want higher quality/lighter you're going to have to pay quite a bit more I'd guess.

Lower limit is typically a fair bit below the comfort rating, so they're presumably naming their down bags after the comfort.

Not they're not at all consistent here with the synthetic ones! For instance the EN ratings for the Lamina 20 are 1 comfort, -5 limit. They're far from alone in this though, but for instance the phantom 32 is only just  less warm than the lamina 20.

(2 degrees rather than 12!!)

Anyhow, what you want to do is calibrate from the temperatures you've happily used the lamina 20 in, and that'll tell you what sort of EN rating you might be able to be happy with for 3 seasons. 

There's a slight problem mind. Any good down bag with ~400g of fill will be at least a little bit warmer than the Lamina and a lot lighter with it. So you're looking at very lighter/minimal options really. Well that or just going for ~400g and replacing the Lamina anyway.

But lots of options, ME, RAB, PHD (esp in their upcoming sale), WM, Golite etc etc.

As you prob know MH give all their bags a figure which is the alleged US fahrenheit rating. I originally was going to buy a PD600 but as Alpkit never had stock or reasonable lead times I got the MH cheap from the states and as a 3 season bag I can't fault it.

Alpkits are meant to be a bit on the slim side I think so if you're a larger person you may want to enquire some more.  

 As Martin says there are lots of options I always rated ME and if buying now would prob get a PHD Minim 400 but they are all going to be quite a bit more unless you can find a deal on somewhere. 

Yes, but the thing about it is that that rating they give their bags is ~10 degrees warmer for their synth bags than their down ones. Absurd, but everyone does it (thank goodness for EN.).

Well yes, but the point about redundancy is there. The Lamina is +1/-5 on their website anyway (-7 iso of -5 is almost certainly within the margin of error on these measurements.). Having actually checked, its simply very similar to a good 400g fill down bag.

For instance RAB neutrino 400 = +2/-4. The Pipedream 400 figure isn't really given as a lower limit but it'd very likely EN test around the same. Real specalists like WM/PHD can do a bit better, but you'll pay for it! (or in fact RABs new outright weight obsessed bags.). 

Ok fine, but these are also ~400g+ lighter than the Lamina, so when would carry both?

All this really means of course is that - if you know the Lamina does you for winter - you clearly sleep warm and can get away with a very light spring - Aurumn bag. Although the very lightest down bags do tend to be premium things, and so you'll pay a fair bit. 

What sorts of price constraints are there on this?

Well MH's naming simply is certainly flat out confusing! For instance the Piute 20 is -4/-10. (if about the same weight as the lamina as not great down.).

There's a bit of a problem here, in that while there are some very light down bags they tend to also go for very high FP down/light fabrics and end up expensive.

A bit like this or this say. Or getting slightly 'extreme' in terms of weight saving: this. Even thing a bit cheaper like this or this won't be trivial.

Really it depends on how you want to balance cost/the possible weight savings etc. That MH Phantom 32 on sale in Cotswolds sounds pretty reasonable really, although even that isn't really cheaper than the pipedream 400. Not easy.

For warmer weather and light weight I'm thinking of this:

http://bac-e.com/sleeping-bags/cumulus/cumulus-quantum/cumulus-quantum-200-sleeping-bag.html