I think the best thing to do when these Disco's develop this fault it to bypass the lot.
Before I owned my Disco the spider was bypassed as the starter feed was not working but it could be bump started - anyway, many years later it developed the issue on the FIP solenoid feed. My engine would turn over, so I sat down with the wiring diagrams and found an ignition switched AND starting position live feed. I found an SRS circuit would be live in run/start position so it was perfect, it was uninterrupted too. On the fuse board under the steering wheel and there is male spade terminal, this is on the SRS fuse, I didn't have SRS so didn't mind taking this feed. This was wired in through an existing grommet on the bulkhead up to the FIP as stock looking as possible with black sleeve etc. and worked like that for the rest of my ownership, I am certain it will still be like that as I know it is still driving about. This saved taking the whole dash to bits and took me about 15 minutes to fit, although the research with huge piles of wiring diagrams printed off took me a while!