Complex sentence: Anecdotal evidence suggests that one of the UK's nuclear submarines noticed a variation in ELF signal strength around the same time, as the MoD files are still classified this is not yet possible to confirm or deny.
Split into simpler sentences: Anecdotal evidence suggests that one of the UK's nuclear submarines noticed a variation in ELF signal strength around the same time. Because the MoD files are still classified, this is not yet possible to confirm or deny.