Hopefully this has been resolved, all the signs indicate it has.
The engineer was brilliant, checked the router which was fine. He then checked every bit of cabling and found the fault to be a damaged network port in the road (it literally fell apart when he touched it!).
He has switched me to a different port which seems to have dramatically improved things - I am still getting occasional issues but apparently this won't be rectified until a network engineer can change the port in the road...although I am unsure if I will be informed about when this will be done.
Thanks for updating us as to how the engineer visit went and happy to hear that things have improved. If he's logged it with our Networking Team, they will arrange for the works to be carried out which includes any necessary traffic management, permissions, materials and so on. At least we know what the culprit is and they will be working to getting it fixed for good.