I read something somewhere once about ringing 17070 to find something out... can anyone tell me what 17070 is?

I believe it is a engineer test number.

BT Line testing. Used to tell you your distance from the exchange a year or two ago.

17070 can tell you your telephone number.

It can also do a ring back test (Test to check if a line is working)

It can do a quiet line test (Sound quality of line)

And a few more things.

Service is free.

Yes that it can, but they dropped the distance announcement as too many people were phoning up complaining they were told they couldn't get broadband, yet they believed they could because of the distance quoted on the service. They didn't realise that distance was probably not the reason for not being able to (at the time) get broadband.