On the login page of the All4 app - I have resolved it by simply changing the password to not include that character, however, as a first time user of this platform, I couldn't find the character. I'd also observe, based on some of the other comments on this thread, that ultimately it is the platform's responsibility when approving an app for inclusion on the said platform that it has been designed to a suitable standard, which would (for me) include ensuring that if it is not deploying a standard keyboard then it must include the full range of characters.