Hi Julian
If you wish to lighten the load and make life easier and cleaner then you should try Kemdirect toilet additive from Holland. www.kemdirect.nl
It is a biodegradable, granular, dry product and replaces both top and bottom so no more bottles needed. Definately less weight.
You can use half sachets if required and it completely breaks down the contents so all you pour away is liquid.
It causes no problems in the rinse tank and in fact keeps it from requiring cleaning and no problems with black bits.
You also get no staining from it as contains no dye so no more blus fingers!
Hans used to work for Thetford and left because they would not produce better chemicals. Kemdirect has been selling for 5 years now in Holland and 2 here in the UK.
It has no smell and there is no chemical smell or other smell in the toilet and no smell when empting the bottom tank.
Hope this helps... Joyce