woah woah woah!
Hold it right there,
firstly I had this problem after the battery went flat - after leaving my lights on- left it at MAF (or whatever position) as I didnt think the indicators would be bright enough to show the car on the road- after running out of petrol dammit!
Anyway, did the garage say why it needs a new immobiliser?
Which garage is it please?
Firstly, when mine was quoted it was £460 fitted with one key - has to have a new key cut too you see. This is for the ECU computer (is there another one then Pud237?)
Anyway I'll cut this short, when the AA man came initially to take the car to the garage, he simply charged the battery, (mine had the immobiliser warning message on the dash too) then he simply took off the red and black battery wires and held them together just like Doc Brown (not connected to the battery of course!) which wiped the immobiliser message and effectively reset the comoputer.
He put the wires back on and hey presto - no immobiliser message so dont need code, and all back to normal!
If you're worried about the computer, your current position is that its apparently knackerd so what harm can you do?
This was after one of those diagnostic jobs where that apparently said the ECU was knackerd!
So try that, or get a second opionion as you shouldn't be paying more than half a grand anyway!
Apparently my ECU and matching Key would've been £410 unfitted.
No way should you pay a grand!
Let me know please what happens!