Yes! Got my head round this. Go to:
iTunes.apple.com/gb/app/gardeners-world-magazine/
Or search in Google:
registering for free gardeners world ipad app
On app screen download the app and then go to:
https://www.buysubscriptions.com/the-buysubscriptions/login
You need to create your account with original email address used when first subscribing. Once accepted or you get an error saying email address already in use, go back to your app you downloaded, clck the blue login. Button and login!
Apologies if everyone knows this, but I found o real instructions and this was driving me mad.