Just thought I would come back and update this thread to let you know what I bought in the end, in case it helps anyone else.
I found someone selling goose down 2-3 season bags new on Ebay and got them for £55 each. They have temp ratings of +3, -2, -18C and are very lightweight at only 890g (confirmed on my scales). They say they have 90/10 hydrophobic white goose down fill at 800 fill power.
They are British made by a company called Hill Venture and I suspect that they have gone out of business and the owner is now selling off the stock on eBay. The original price was £200 each. They were listed for £65 buy-it-now or best offer. I started with an offer of £50 which was auto-rejected, then went to £55 which was sent to the seller and he accepted. They arrived very quickly and I have to say that I am very pleased with them. I haven't tried sleeping out in them yet so I can't say how they perform, but I think with a liner and merino base layer they should be good for most situations. I couldn't find any reviews online or evidence that the temperature test was carried out officially, so it did feel like a bit of a punt to buy them, but now they are here I have to say that they do look and feel the part. Now they are here I can see that the temp label does say 'EN 13537' on it. The guy hasn't done a very good job of marketing these on his website or eBay.
They have a YKK left half zip with a thick baffle, filled drawstring hood and a neck baffle. There is velcro to allow you to join the two ends of the neck baffle together once you are in and also a small 'valuables pocket' built into the neck baffle. This just snugly fits my phone (Moto G4) but doesn't close with it in. This might be a good thing as it means I could take the phone out quietly to check the time without the velcro fastening waking the boy up.
They arrived in a stuff sack, but with a quick shake they lofted out pretty well.
If the temperature ratings are correct then these should be equivalent to something like the Mountain Equip Titan that Stu linked to, or a couple of centigrade poorer than the Alpkit Sky High 500. For the price that seems pretty good to me.
http://www.hillventure.co.uk/product/agate-sleeping-bag
http://www.ebay.co.uk/itm/Hillventure-U ... SwPCVX-1Y0
The guy seems to list them in batches of 3 on Ebay, though his website says 56 available.
In the stuff sack:
Lofted out:
Opened as far as it goes:
Neck baffle detail:
Temp label:
This is it inside a Alpkit 13l drybag. It needs other stuff in there too really as the bag is too big otherwise, but it is just to show the size. I haven't done the straps up very tightly: