Here's my shilling's worth - although it may not be worth quite that much. The advice so far has been ace and from people who know what they're talking about but I'll add my findings anyway.
I did all-year biking for 2 years on a 28-mile each way trip to work. After 2 years I realised I wasn't enjoying it anymore and reverted to being a soft lad.
Unless you spend big stylee I think you'll find that waterproof stuff eventually isn't. For me, a pair of waterproof socks (sealskinz etc) was invaluable. With thin socks underneath they were reasonably warm and you buy knee-length too. As Juve said, I found waterproofs (2-piece) over the top of my textiles to be great for doing what they say on the tin and reducing wind chill. I find that, for some reason, most manufacturers have designed thermal liners to finish about an inch before your zip (ie they aren't present where a draught is likely to be) so that extra layer of wind protection helped.
Several thin layers - preferably thermal - helps too. Neck tubes - 2 indeed! But I'd add a third layer of one of these
http://www.getgeared.co.uk/probiker-neck-and-face-protector?sc=21&category=403570. Although I wore mine (from Aldi or Lidl) under the nose, otherwise I blew hot air straight upwards. I'd have a neck tube under, down inside my shirt; then the windstopper thing outside my shirt but inside my thermal top; then another neck tube over the top of the lot and over the top of my head like a crazy ninja biker.
Gloves - 2 pairs as stated - waterproof, with more cloth than leather as they dry quicker. I usually go a size bigger in winter as tight gloves are hopeless in the cold. If they're big enough for a pair of liner gloves then that's even better.
Muffs, no - simply couldn't get on with them. Heated grips - no - they warmed the inside of my hands when the outsides were getting cold. Although, I didn't have hand guards to fend off the icy blast. I have heard great things about heated gloves - heat where you need it - but they were out of my price range. I have also heard good things about the lobster style 2-finger-plus-thumbs gloves (eg HG Pathan) but gave up before I got a pair.
Mirror extenders - no. I have a pair now but that was more about airflow on my K2 Vee, although the view is good. For me, commuting involves filtering. I really don't understand bikers who just sit in a line of traffic, although I recognise that's their prerogative. Wide mirrors don't help me squeeze between vehicles. Standard mirrors work fine when I'm using them properly and, on the occasions when I wasn't, life savers have don't just what they were supposed to do.
Finally, the helmet. The pinlock system is one of the best biking inventions. I wouldn't go back to a non-pinlock helmet. An internal sun visor is also great with the in-out nature of the sun in Britain (even if it mostly in).
