Thanks, but with the kind help of @duncs I think we've ascertained that the cable itself is knackered/the connectors not installed quite right.
In the meantime, while I rage at the electrician, I've managed to use the access port in range extender mode and got a mostly useable speed until it's fixed
Trying to crimp RJ45 plugs onto cables is a waste of time. Punch fixed cables onto the back of sockets, it is so much easier and so more likely to work, and then buy some patch cords.
+1 to this. I've done a few for my neighbour downstairs and redoing the punches into sockets is considerably easier than crimping plugs onto the cables - he had a mixture of both.
A full rewire including Cat5/6 cabling is top of the house list and is just awaiting the money.
Thanks, but with the kind help of @duncs I think we've ascertained that the cable itself is knackered/the connectors not installed quite right.
In the meantime, while I rage at the electrician, I've managed to use the access port in range extender mode and got a mostly useable speed until it's fixed