Installing FCP on a Clean El Capitan iMac fails everytime

Looking for some help please?

I've recently formatted & rebuild a 5k iMac to solve mac mail issues. Over the past three hours i've attempted to download and install FCP with no success. Firstly the app store download button says [Update] rather than [Download] and after 17 minutes the download fails with a pop up suggesting i go back to the purchase page and try again. Other large apps like Logic Pro have installed without issue. Any help would be very welcome.

Regards Marc