I'd suggest that you're pretty well set already, just throw an underblanket into the mix. I've used the DD Underblanket in temps down to -2 and it works well enough. As per the above comment, it can be a little bit bulky, but does compress down reasonably well (tiny bit bigger than a Nalgene bottle). Pair that with a Carinthia D4 and you'll be snug as a bug. Like me, I see that you're in Kent, we're fortunate in that we tend to get off lightly with the winter weather and don't experience particularly extreme temperatures. I usually rely on my 3 season sleeping bag through the winter time and it has worked fine with the DD underblanket.
You might want to think about your tarp though. I expect you need to pitch the basha in a diamond config in order to cover the hammock and some of the suspension? Have a look at getting a DD 3x3 or better still the 3.5x3.5 for more protection and living space. You'll appreciate it when the weather closes in and you're tarp bound. The larger DD tarps are also a lot less bulky than the basha's so there's another bonus to getting one.
Enjoy, this in my humble opinion has to be the best time of the year to get out camping!