I tested now my 1 year old Nemo (it's 44-45) with a normal shoe sock, I mean normal cotton sock, not diving sock. With a normal sock it's too loose to my 44 foot. I would say my 44-45 Nemo foot pocket is as big as a 45-46 EU shoe. With 5 mm neopren sock it is ok to me.
So I think 46-48 can be too big for you even with 3-5 mm sock, and 44-45 would be maybe better. First (if 44-45) it would feel a bit thigh, but in Nemo the rubber in foot pockets is soft and streching, and with a diving sock it will be comfort, I belive. If needed, you can first use a little soap or shampoo to get a sock to the foot pocket, or put first sock to the foot pocket and then soaped foot to the sock.
But if you take 46-48, you can surely use 7 mm socks with it, but maybe not 3 mm socks. It think that's not a problem to you in these Northern waters :blackeye
Edit:
A clip from an other monofin thread:
I have a model 1 that used to be torture on my feet. I use a lube (liquid soap) and that helps. The more I use it, the better it feels. I don't think it will ever feel good though. I was given some advice from a very accomplished finswimmer about footpocket comfort. If you can identify where the pocket is too tight, soften the footpocket slightly with hot (not too hot) water and stretch that area with a tennis ball or similar object.
I also have a waterway glide that is also extremely tight.
With either fin, The most I can use them is about 15 minutes before my feet get sore.
Maybe, Gordon if your really prefer comfort and don't want risk that, take 46-48 and buy 7 mm socks if 5 mm is not enough, and you surely will not have any of these problems. WW Model 1 and Glide foot pockets are not so big and comfort as Nemo's, anyway.
Disclaimer
But as you said, better to tell in your order to WW what you want (comfortable freediving for hours with 5 mm socks) and what are your measurements.
PS.
Gordon, if you remember, please tell us afterwards, what you get and what were your testing results with the monofin.
To get the right size to you monofin is so typical problem to freedivers. And Nemo is one of the most sold Monofin in the market for long lasting outdoor use, where comfort is essential.