Recommended Waterproof Cable Gland for Ethernet Cable?
Converting this from a comment to an answer:

I suspect the solution is not to even try. Just take a plain connectorless cable (or cut the connector off your existing cable), pull it through the cable gland and then crimp on a new connector.

As Kris Bahnsen notes, cable glands are not designed to have bulky connectors squeezed through them.  If they were, the wouldn't have anywhere near as good a seal against the cable.  The fact that you've found just one part in the Farnell catalog that has almost the necessary size range, and needs extra reducer rings to achieve it (BTW, good luck getting that 5 mm ring over the connector), pretty much indicates that what you're asking for is at the very limit of what's possible, if not beyond it.

Using a waterproof jack and connector, as Kris suggests, is one option, and has the convenient feature of letting you disconnect and reconnect the cable as needed; with a cable gland, the cable would be pretty much permanently attached to box.

However, if you don't need that option, I'd still say the easiest option would be to use a normal ~5 mm cable gland and crimp a new connector on the cable after threading it through the gland.  Honestly, people do that all the time, because it's the only practical way to run RJ45 through tight spots (or to get a cable that's exactly the length you want, rather than one of the limited range of lengths available pre-cut).  Just make sure to test the cable properly after attaching the connectors, and repeat if necessary.  It's not like the connectors cost much.

By the way, I hope your cable's also rated for the conditions you're using it in.  In my experience, the outer sheath on cheap Cat5 cables sometimes loses flexibility over time and may crack under mechanical stress.  That's generally harmless in an office / data center setting, but it could be a problem if you need things to stay waterproof.