Well I phoned BT, and the vans were gone the next day! They wouldn't tell me what work was going on, if any, only the status of my own phone line.
But on Saturday evening there was a very suspicious young man, about 35ish? Wearing jeans and a shirt, driving a posh car. He pulled up outsid emy house, went over to the manhole covers and kicked the dirt around the edge (they're on a grass verge so are slightly inset in the ground). Then he got out a clipboard and started writing stuff down. He looked up and down the road carefully, then at my house specifically, then wrote some more down. Then he opened the mahole cover with a stick thing, looked inside and made more notes. Then he got out a wheely thing and measured the distance between the manhole covers. Then he got back int he car and after waiting a little while, he drove away again . . .