Not starting - possibly immobiliser?

Hi all,

I'm tearing my hair out here and really need your help please.

I purchased a K11 2001 last year with 63,000 miles on it. It's been garaged for the majority of its life and the body work is like it's just come off the assembly line.

Long story short, I've been having trouble with various engine codes and having come to the end of my tether I took it to a mechanic and asked them to try to figure out what the issue was. After a most of the day and a considerable bill they told me that the ignition, injectors and cylinder pressures were all fine and they started looking at the wiring which was... a mess...

So having reached the end of what I could afford to pay them I brought it home and started investigating myself. There was some bad soldering and pinch connectors (sorry if that's not the right term) within the ECU loom and an extra wire that was running from the loom in the engine bay to the ignition switch. I did my best to repair the poor connections but alas, I had actually made it worse.

So I found a breakers that had a wiring loom and today I have spent the day removing the old loom, including the extra wire, and putting the new one in, which was in very good condition except for the pre-cat o2 sensor connector which I have swapped for the one on my old loom.

But now the car won't start. The engine turns over and the spark plugs are sparking but it won't fire.

So I'm now thinking that a previous owner has had trouble with the immobiliser and made a hack job of bypassing it. By replacing the loom and returning it to factory standard, I've gone and reintroduced the original problem.

So my first question, does my conclusion make sense?

If so, what's the best option from here, try to find a ECU, key and barrel from a breakers?

Any advice greatly appreciated, thank you!
Does the check engine light flash? And does the led on the dash near the heater controls go out? Or does it keep flashing?
And have you cleaned the connectors next to the battery?
Hi Maarten,

Thanks for the reply. When I turn the key to ignition the engine check light is solid on and the led be the heater controls goes out. Which connectors are you taking about near the battery please?

Will do, thanks Maarten. Could you tell me what the engine check light and led by the heater controls has told you please?
The led that goes off means nats has detected the key and that nats prolly isn’t your issue. The check engine light means that there is a issue and you should get a code reader to check it. It used odb2 so a cheap reader will do fine (elm327 for example)
The check engine light isn't flashing... doesn't it always come on when the engine isn't started?

I have a reader, and it's not showing any codes right now... Will check the fuses and go from there. Thanks again for your help.