Frick. On a stick.

Ok, car didn't start today and it had the tell-tale signs of a dead battery. Bought a new one from Halfords (rip-off but wanted it quick) and fitted it.
It didn't start up 
The starter motor goes strong, unlike befoere, but it doesn't turn on, even when there are some rumblings.
Computer says 12 miles range left.
Did paperclip and got
0335 = Incorrect RPM Signal
0203 = Injector valve 3 (been a rare intermittent problem before)
1502 = Immobiliser No Signal

What should I do?
How painful is this gonna be?

You may be out of fuel too if it only reads 12 miles.....  drop a can of petrol in and see if it starts up

2.0 16v to 14194716 90458251 £60.50 £54.46 £47.10

 
2.0 16v from 14194717/31000001 9174621 £36.60 £33.00 £28.47

Remove heatshield, remove dipstick tube (push-fit) about 50mm forward of tube hole lives the crank sensor held in by same size torx as heatshield. If you can`t remove by feel alone, remove A/C compressor bolts & swing away for clearance
 

Thanks peeps. Put some petrol in and no luck so it's not that.
Apart from getting under there with a magnet is there a quicker way of knowing if it's a steel or aluminium sump? I know it swallows a 5 litre bottle of oil when refilling that takes it halfway up the dipstick.

I'm not too sure why you need to know, but I think the ali sump is a quite different shape to the steel one.

This is a steel one:

Thing is, Jimbob has already mentioned buying a genuine sensor, Rob G has given you the part numbers by engine number, and the engine number is on the V5 document, I do believe.

That's all you need.  Unless you want to gamble on buying a pattern part.  Note that although they use the word Vauxhall very liberally in their descriptions on that vauxcentre site, when you read the FAQs, the question "Are your parts genuine?" is answered "Yes and no..."