I can comment.
I bought some 46s this week - went up a size as you did.
Wear thick socks - just got back from first ride wearing those silly thick Heatholder thermal socks.
Lovely and toasty and crucially still wiggle room for toes. Admittedly it's not cold now, nor was it raining but they do feel lovely. Try thicker socks/insole first before selling.
If you do go down that route get them from Go Outdoors - I got mine for £99 as they price match then beat that by 10%.

[s]Try thicker socks?[/s]

Sell them to me for 30 quid?

I sized up with my Celsius Artics and fitted a pair of [url= http://www.superfeet.com/en-gb/insoles-and-sandals/merinogrey ]these superfeet merino insoles[/url], very comfortable and very toasty - they now fit perfect.

Mine are 44, same as my MTB and road shoes and trainers. Bigger than 'proper' shoes.

I have the Fahrenheit mtb boots and found them a bit big. I am around a size 7 and normally wear size 41 road and mtb shoes but found that a size 40 northwave fitted me better with plenty of room for woolie boolies

I'm a 9 and the 43 fitted me pretty well but after many years of winter boot experiance I personally find that my feet get considerably warmer with a little extra wiggle room so I've just bought a pair of celius' s in 43 which are great on the bike

My Celsius Arctic's are 45 (I'm 44 in trainers 45 in cycling shoes) and are a bit too snug if anything

Are they the mtb version or road? I've got a pair of road ones in a 45 I think are slightly too small for me that I haven't used but bought them just before the summer so can't return them and think I'd prefer a 46.

I'm a UK 10, but just sent back a pair of size 44 as they were too tight. They're a stiff shoe compared to my ancient Shimano's which may have contributed? Either way, figured a bit of literal wriggle room in a winter shoe would be a good thing, so back they go.

Bit of an update.
After one dry road ride I thought these shoes were great.
Having done a wet Lakes ride yesterday I ended up with wet (but still warm) feet.
I had waterproof baggies on with leg warmers which were over the top of the boots but aren't waterproof.
There were loads of puddles etc and a few river crossings - I cant be sure at what point they got wet - would like to think they were fine prior to that. Anyone get wet feet using these?

I bought a 42 but wish I'd gone for a 41.5 as there's loads of room. Never been an issue though, thick socks and they've been toasty. Even when wet. water gets in from the top, but doesn't seem to chill feet like I've has in Shimano boots.

drysuit seals worn round the ankles and overlapping the top of the boot will stop water getting in from the top. I think the boots themselves are waterproof the big hole at the top is the obvious weak point with all shoes.