Usually a faulty Ethernet cable that's causing the hub to drop back to 10/100 ethernet (had exactly this myself last week), or it could be the computer has a 10/100 network adaptor. Tends to be only old or cheap computers that still have 10/100 adaptors. If buying a new cable, they're cheap as chips online, and I'd say go for Cat 6a or higher.
Thanks for your help, I swapped the cables to try a few different ones. Same.
But I since think i've found the culprit....I'm an idiot and didn't check the age of the 16port switch. It looked pretty old but assumed that it'd definitely be 1000mb..... Though on closer inspection it looks like it is not.
It's a 3COM Baseline 2016 - Model# 3c16470... link below.
Yes, Cat 5 will normally only accommodate 100 Mbps. Cat 5e will do up to 1 Gig, but it may be pushing your luck on longer runs, and bear in mind the possibility of noise and cable/connector ageing on built in cabling. The more switches and junctions in the network, the more chance the hub will decide to limit connection speed. And as per my own findings the other week, a cable that's nominally 5e or better ( which is almost any you;d encounter these days as basic Cat 5 was deprecated as a standard back in 2001) can still go bad, and then will sometimes flip between 1000 and 100 Mbps.
Looks like i'm in luck with the cables atleast. They do appear to say cat5e on them.
Though my other question thats arisen, is.... this is a 16port switch right? what are the two rack mounted things above it?
It looks like 12 of the blue cat5e cables are going from the switch into the rack mounted thing on top, and all the red cat5e are running from the middle mounted rack to the top mounted one. Then I assume out of the back of the top rack mounted ones are the grey cat5e cables that are feeing the ethernet wall sockets in each of the other rooms in my house. Images below.