Have you punched your vin into ETIS to see if an immobiliser was actually fitted to your car?

 

Hi Gus.

Thanks for that..
Something else to check...

 

I'm beginning to think the exciter ring and immobilizer are one and the same.

 

Hi Nick,

I also have a 1998 XKR and went through a similar no start problem some years ago but without your engine noises. Anyway having been soaking up all the information from kind members on this excellent site for many years as a silent viewer I thought I would join this one as the simple solution to my problem cost zero and the fix took 2 minutes, I thought it might help you to make this simple check.

I established I had a spark and fuel pressure but it would not re-start after a 45 minute run, tried to do all the things you are looking at and no success.

Look under the bonnet on the passenger side, lift the large fuse box cover, in there you will see another black plastic cover that looks a little like an inverted cigar box. This cover is clipped in place but will lift straight out. Inside you will see three or four relays, one is fuel injectors, one is sparks I don't remember what the others were, anyway on looking at mine the fuel injector relay had moved about 1/8 inch out of its receptacle I pushed it back into its seat and the engine started straight away. I put this down to potholes due to the diabolical state of the roads around here. Since then I have come across 2 more XK8's that had the same problem and fix.

Peter