As promised here is a photo of the hole I cut in the panel above the tank, I had other photos but seem to have lost them but at least it gives an idea of how I got into the tank.


Due to the angle of the panel above the tank the hole looks offset to the access plate but it was directly above.
Also I mentioned my level sender was only applicable to 2000 - 2002 models and I had to replace it by feel as I could not see where it fitted. I had to go back in the tank and try again because it was giving too high a reading. The square plastic plate actually slides into a narrow channel at each side and locks in place when it is fully home. Easier to understand when you see it but that photo has vanished.
I have now worked through some other issues with good results. The rear quarter windows didn't seem to operate correctly but with a squirt of lubricant they seem OK now.
After getting everything functioning with the replacement SLM I decided to check what would not work with the old module. It turned out the only thing not working was the ability to programme the remote fobs. I thought about this over a few days and beers and wondered if it could just be the little RF receiver board inside the SLM. I looked on Ebay and found an SLM from an X300 and got it for 16.50 with postage, so nothing to lose. When I opened up the X300 box the RF board was noticeably different but had the same ribbon cable connection so I swapped them over. Today I tried it in the car, lo and behold everything functions, the remote fobs programmed as they should. So if your remotes stop working and you can only set the alarm by locking the drivers door, check the little RF board in the SLM, The RF board might be the same in most Jag SLM's so you should be able to pick one up cheaply. I now have a spare SLM which I will keep just in case things go wrong again.
As it appears to be my lucky day I bought a lottery ticket.