You may need to run a software update to properly activate the BT Player and its button, the installer should have done that.
On the wiring, power line adapters will work but he should have fitted a new cable and I'd suggest calling BT to complain. OTOH your wife presumably signed the job sheet and/or failed to raise any objection at the time. Agreeing the work is acceptable is why BT insists on an adult being present.
Thanks 4KMarty, my wife says no job sheet signed. She left the chap to get on with it and five minutes later he said it was done, showed her BBC1 on Freeview, said the box wasn't doing updates but it should do them "in the next few hours" (8 hours later nothing seems to have happened on that front eg BT Player button still doesn't work).
Also the really long gigabit cable that came with the router seems to have gone completely.
Can't be sure of connection quality yet but iPlayer is a little choppy i.e. will occasionally buffer.
Speedtest on laptop via wifi gives a beautiful 48 down 13 up so no problem on connection speed.
LaurenRachel - that would be great BUT I signed up via Topcashback and in their T&C's to get £155 cashback it says the cashback will be lost if I contact BT about the order....?
By default the box will only check for software updates overnight (it's about 2-4am) to prevent your viewing being interrupted.
The updates check for new software - though this should also happen on first activatation - and also any updates to your subscription to activate the extra pay channels / BT Player.
You can force this by going to the settings menu and updating manually. However if the install was only today it's possible the order won't have completed as yet and not everything will activate until it has.
I don't recall a very long cable coming with mine TBH, I use powerline adapters and they work fine but if you paid for an install you're entitled to an install.
Quick edit: Re the BT Player - if you press the blue (Y) button so you see a BT icon on the onscreen menu? If not, or if you do but it's dull and cannot be accessed, this means the box needs to do the software / configuration update discussed above.