Thanks for responding so quickly Trevor!
The other company is based in the UK (is VAT registered), and the contractual link to the app store/users is with the other company only. I’m not sure if this is relevant but it’s also my understanding that the payments from the app stores do not include any form of VAT.
Regarding my supply of services to the other company, I am not invoicing them for any services supplied, but rather receiving a % of sales. Additionally, there may not be a continuous supply of service (e.g. further work from myself on improving the app), not sure if that makes any difference? The lack of invoicing from my side of things, is what made me question my VAT liability in the first place.
Thanks again.