Phoned up BA for the alternative airline locator for LATAM. Got it with no problem at all. Have now booked seats!
I have several flights booked with avios via BA site. 2 came with BA booking references that had 5 letters and a number. ( didn’t work on LATAM hence the call to BA) The third flight had a BA booking reference of 6 letters ( no numbers) and worked OK on LATAM site. So perhaps it’s the presence of the digit that causes inability to directly check booking on LATAM MyTrips