Got the kit Chris thanks, done a few winter arctic camping trips, I use a tarp or build a shelter, so have the keeping warm part generally, covered. I think, reading the helpful replies, that I just have to accept that £200 will probably not get me a decent down bag. Thanks again, as I said, time to sell another knife!!What kind of camping do you do? A super warm bag, for a special trip north, might be tempting, but will you really be able to make the most from it? Also, what have you got to sleep on top of? I haven't been really really cold (-15 to -20C only), but even at that level good under insulation was really important. On top I used a Marmot Helium bag and a Sierra Sniveller quilt in combination. I can use either one by themselves in warmer weather (by which I mean, in a hammock, down to freezing).
I think that you may be optimistic about what you can get and budget. Just looked on ebay at used down bags, and even used, the good ones, and or the ones for deep cold, are more than your stated budget (looking at new bags).
https://www.ebay.co.uk/sch/i.html?_from=R40&_sacat=0&_nkw=down sleeping bag&_dcat=87100&rt=nc&LH_ItemCondition=3000&_trksid=p2045573.m1684
Not much help I know.