I suspect that SummersSquad‘s assertion that this should be archived is incorrect. The area around the old swingbridge has been fenced off for some time (it was when I logged the cache, last year) but this does not affect the ability to find the cache. The cache description already makes very clear that the cache is not on the swing bridge.
I’m reasonably confident that the GZ cannot be contained within the fenced-off area, as this would also prohibit either access along the waterway (which would be listed on the Canal & River Trust website, and it isn’t) or else would involve the closure of the nearby road bridge (which would prevent road access to the Northern end of Rewley Road, which seems unlikely given the number of people who live down there!).
I’ll try to visit the site within the next week to confirm my suspicions, but for the time being I’d suggest NOT archiving this cache.
(I’ve sent a message to SummerSquad to clarify the situation)