Simon,
We got fed up with the black bits constantly returning,so went onto Fenwicks which can be used for the top tank as well as the cassette.Smells quite pleasant and we've had no bother since.
Go on to the Fenwicks web site to read all about it.
There is also a Dutch product which Joyce keeps telling us about,in granule form which has no smell at all......but I've never seen it for sale in the shops.JOYCE,can we only get this through you??