Any ideas?

So I was avoiding a deep pothole on a country lane and hit a deeper one on my left hand side hidden by water. I was doing about 50kph so it got a good bang. Engine management light came on and when I pulled up it would idle really low and then high and kept going up and down. It now splutters a lot in 1st gear but in 2nd-5th it's fine. Any help as to what would be causing this would be great.

I'd check all the coil leads the bang might have dislodged something

MAF would be my first guess there... they are fragile and you described a big impact followed by erratic running (sounds like fluctuating mixture from your description of splutter and high low idle) and engine light on...
Would get the code read asap and drive her very easy as a bad MAF can damage your engine very quickly.

Other ideas maybe if not MAF...
Idle control valve?
One of the vacuum lines getting false air?

Id check all the hoses/lines first, then get the codes read. Its strange its better in the higher gears but could that be because youre not hitting boost as often there?

I'd say keadeen hit it with the maf

Have you an induction kit?

Check plug on coil packs. Clip can be brittle and bang may have unclipped