To provide an update - an engineer visited on the 26th and said the line seems fine but he saw it dipping from 27mbs/s to 4mbs/s and also saw the line drop out completely. He said it is most likely a broken/shorting/corrupted connection and whenever someone else uses something on the same circuit (excuse my terminology, I am not an expert) it is shorting with ours. He did say it could be interference from something outside the house as well, but most likely a damaged cable.
He was a really helpful and polite guy and booked an Openreach appointment for the next day and said we may not see the engineer, but they will be working outside or at the exchange to resolve the issue. The next day we checked our speeds and nothing changed but we then had an update that the issue 'should be resolved by 6pm on the 29th'. We checked all over the weekend and it was still dipping up and down but on Sunday it went up to 30mbs/s which is the best it has ever been. However it is still dropping to 1-4mbs/s at night and this morning it is running at 4mbs/s. Seems like they've made our overall speed better but it is still dropping as it has been for the last 5/6 weeks now.
I went on the fault tracker and it says "you have told us the fault is now fixed, thank you for confirming" - which I never did. It is also telling me that as the fault has been fixed and it is all now working just fine, that I will need to open a new fault...
So - I am just about to phone them again and see what they say, but that is where we got to.
