Plenty of things in the house could be causing it, including the PSU. If you want to be 99& certain, then turn off EVERYTHING including fridge/freezer, heating, alarm, TV, sky box, any chargers &c. Leave just the circuit being used to power modem/hub and just that and te PC connected. See what happens then. It is extreme, but it will remove a lot of doubt,

Do you know where your cabinet is? How far is it by road? A rough approximation will do.

Yes, the cabinet is 400m away.

It is a freshly built cabinet as is the one in the exchange, it was only activated a couple of weeks ago. The openreach engineers when they were pulling the fibre cables etc mentioned that we should be looking at 60+Mb.

At 400m 60Mbps is normally easily achievable. I am around 440m with a line attenuation of 16.5dB and have a max attainable of around 76Mbps - if someone about 4 houses away turns their equipment off, mine jumps up to over 80Mbps.

In the 60s I would have just said "a very noisy line and hard luck" but in te 40's almost certainly a line issue.