I really struggle to stay warm on dives this time of year (thanks a lack of bioprene), so much so that even with 3 undersuits and dry gloves I am still struggling around the 30 min mark in 6C water.
To counter this I bought a Typhoon Heated Waist belt from the Stoney shop on Friday thanks to Bardo's recommendation (at the bargain price of £99 - cheaper than online). I used it this Sunday and it makes a hell of a difference. I was noticeable warmer on Sunday than I was on Friday - but the 2hr burntime on the battery is a restricting factor if you want to do two or more dives in a day.
Having spoken to 4 different Typhoon dealers in an attempt to buy a 2nd battery for it I ended up going straight to E-X-O2 and it appears that their Stormrider battery pack is the same as the one used on the belt and be bought from here:
StormRider Heated Clothing Products - EXO² Heated Clothing and Heated Motorcycle Clothing UK
If you are suffering from the cold but adding more layers will restrict your movement then the Typhoon belt is worth considering and a second battery will enable most people to do 2 big dives in a day.