It sounds like some kind of administrative error or registration error i.e. the domain name and hosting should all be with them. Perhaps you registered the two separately when joining up? In any case I'd escalate it with their customer services, you have everything you need to have with them by the sound of it and you aren't trying to transfer anything into them from outside. I can't tell exactly where the problem occured but if you did register the two separately with them, instead of one account, tell them you made a mistake and could they combine the two?
Depending on how long ago you registered the accounts etc. you should have a cooling off period I would've thought so worst case tell them you'll be wanting to cancel everything then start again with them or someone else (fasthosts are pretty good when they aren't suffering from potential security breaches!)