Thanks Noel. I just received an email from support saying this exact same thing.
This is an extract from that email,
"Hey Wayne,
Angela here, thanks for getting in touch.
Glad you're trying out the app!
Unfortunately, there is no way to purchase individual masters within the app, so it really works most seamlessly if you're subscribed to one of our plans.
If you'd like to purchase tracks individually, you'll need to stick to our website.
Once you upload a song to LANDR.com, go through the mastering process and get to the part where you choose your format, you can select which one you prefer and then purchase that master on it's own.
Hope this helps!
Best,
Angela
LANDR Rescue Squad"
So, I'll give it a go from there. Thanks again for the help at this end.