My mistake, being a new iPad user.
The on-screen icon was in fact directing me to double click the physical on/off switch at the side of the iPad.
Hope this helps and explains.
When “getting” the app the install (double click here) button in the corner of the screen is not sufficiently accessible to be able to click it. Only about 30% of the button is available and this is not enough to be able to activate the install.