Don't offer an open account altogether. 

The fact you're offering this existential limbo between an open account and a "trial" is the reason why many people are packing up shop and leaving.  I would say either make it free or a 30 day trial; but from how multiple requests have been given to labels from Spotify to "give us time, it'll make money soon," the metaphorical foot may have to be put down on this situation.  Beyone this, remove the price tiering, and offer one universal service for a cheaper price than £9.99.  Talk up the fact that unlimited music is the price of one album; but anything larger than £5 is considered in the public view as more than a microtransaction.  Price low and go for volume, while eliminating audio advertising but keeping visual.