OK. Back home.
So, first of all, thanks to each and every one of you who offered some ideas as to what it could be. Each one was considered and eventually ruled out stuff that it could be. I hope that people read this thread and may understand a bit more about their vehicles and be of benefit to you.
So here's what we did. I initially presumed battery, so got my charger and hooked it up, no success. I tried that spray that ignites your engine to get it going, that didn't work at all. I tried defrosting the diesel filter and refilling it with diesel and putting it back, and that made a slight slight difference, especially when the manual pump (the mini rugby ball thing) was was primed. Therefore, it suggested fuel.
The battery had been charged, and the charger and a starter kit said that was fine, although after trying the engine a few times, the battery took a pounding.
With the problem being obviously cold-based, I thought the only way it would fix itself would be to heat it up. Tried using a paint stripper (at a safe distance, I wasn't blasting away the engine!) to heat it up, but it wasn't enough. Just around the corner from me, the road runs north/south and today the street was covered in sun.
My mate came round and tried turning over the engine, and when he stuck it in first gear and turned it over, the van moved a few feet at a time. Using this and me pushing, we got the van round the corner, lifted the hood up and let the engine soak a few rays.
After an hour or so, the clouds came and I gave it a try before the temperature dropped. It started up but was groggy, pressing the accelerator did nothing and there was dark smoke coming out the exhaust. Given the fact it had had two days of turning over, that's not a suprise. I ran inside to get my manual just in case, and it had already died.
I waited a few minutes and tried again. This time it started a lot better, almost normal. So I let it tick over for a few minutes, tried a gentle rev and it was like clearing a smokers lung after a night out, which was expected. A few miles of driving around proved OK and I ventured a bit further, then took it on the motorway for a gentle 50mph spin to warm up and clear it.
So far, so good. The turbo's a bit sluggish and it's probably about 90% as it was, but I expect that it should clear itself out soon. Will whack some fresh diesel in tomorrow, and will be doing lots of short journeys tonight to keep it warm ish. Fingers crossed it'll start tomorrow.
So there we go. Moral of the story is don't be such a cheap b@stard and pay that extra for Homestart, but not with the AA - I'm going to go with Greenflag.
So again, thanks a lot for your help, it was good to get a lot of different ideas as to what it may be. I'm fairly OK with trying to sort problems with petrol lawnmowers and stuff like that, but big vehicles are another thing.