You seem to be a bit late. This is from Friday's Daily Express:
"Coverage of Canada and Great Britain begins at 8pm GMT tonight, online and on the BBC red button, before switching to BBC Two at 11.05pm.
BBC Two will show the Davis Cup live between 6pm and 8.30pm on Saturday. Sue Barker will present coverage of the second day's doubles match.
On Sunday, there will be online and BBC red button coverage from 5pm. BBC Two will show the highlights between 11.15pm and 12.45am.
All rubbers will be covered live by the BBC, either online, on the red button or on BBC Two"
http://www.express.co.uk/sport/tennis/762870/Davis-Cup-2017-dates-how-to-watch-when-on-TV-list-of-matches-times-rounds-countries