I'd make sure I was out and plan to return home at exactly 6pm. I'd also leave a note on the door saying that you will be back at 6pm to be there at the agreed appointment time. I'd probably then sit in my car 50m up the road to keep watch.
(You might get the impression that I don't trust BT!)
I recently was waiting for a BT engineer to turn up and by by 2 PM and no sign phoned BT to be told they called earlier but nobody was in. Lies I said! They called the engineer who promised and promised that he had called but we did not answer the door to him and best they could do was re-arange a whole week later! Grr
One week later ......
Phone call from same BT engineer who insisted I was not in the week before and he was on his way but where did I live exactly?!?!?! Seems he went to totally the wrong house, villiage and post code the previous week even though he had exactly the same details back then too. Muppet!
Trust BT? LOL, only a complete twonk would!