Intermittent Broadband fault on two separate lines/ISP's

I have been having an intermittent broadband fault at my work for the past few weeks so decided to get a second broadband connection with a different ISP on a different telephone line and today the fault occurred again and both my broadband connections went down. Let me explain the problem in more detail...

For the past few weeks I have been losing my internet connection. On my router I have a DSL light and Internet light. The DSL light is always on so no problem with the line. But the Internet light sometimes goes off and stays off for quite a few hours before coming back on. I have tried everything when the light goes out. Re-setting the router, replacing the router, changing the filters, nothing works. It just seems to come back on its own a few hours later. Have had BT in to check the line and no faults found. Have had independent telecoms engineer to check the cabling and no faults there.

So I decided to get a second broadband connection with a different ISP on a separate phone line. Since the second line went live I have not had any problems until today when the internet light went out again. I then tried to connect to my other router and the same problem on that router.

Both routers went down at exactly the same time today which was about 9.30 this morning. That was also the time that I arrived at work and opened up and turned on all the lights etc. So I am wondering if that is a coincidence or if perhaps there is some kind of electrical fault in my building interfering with my 2 broadband connections? However it does not usually go down first thing in the morning. Other occurrences have been at other times during the day and at night so I really am baffled.

Any advice would be greatly appreciated.

A nasty red-herring?:
a charity I have some involvement with changed the location of one of it's offices at the end of last year. BT managed to tie that office to another, as a result of commonality in the requester name, and cut off the line and get the internet (not provided by them) stopped on the wrong office.
They later compounded the offence by making a thorough mess of getting a new line in (more tying together of, to them, apparently connected data) and delayed the provision of the requested service by some 4 or 5 months.

The DSL light is always on so no problem with the line. But the Internet light sometimes goes off and stays off for quite a few hours before coming back on.

By any chance is this a TalkTalk line?

No, they are not TalkTalk lines. Some additional information to add. I have spoken to our my door neighbours to find out if they also lost connection at the same time but they did not so I assume there was no problem at the exchange.