Trick for cleaning plastic bottle is toilet cleaner then half fill with warm water and using a handful of nuts bolts and washers and self tapping screws shake vigorously in a a rotary movement
Rinse and repeat
Of course make sure lid is on and tie a knot in rubber hose
I recently de-crudded my Citroen's cooling system and found a reverse flush helped shift everything out. I also used a little clothes washing liquid (Ariel non-bio, I think...) which shifted even more muck. I rinsed it out for ages and added the mixture using distilled water and pink anti-freeze (50%/50%) and after bleeding the system I've a car with a heater I could fry eggs with and an engine that runs much more happliy! Good luck with your cooling experiments!
Few miles added to the clock but no further work undertaken.
Minor electrical second fix to do.
All quiet with this vehcle.
Hillman Imp now sold so one less to worry about.
Need to get stuck in to the Husky!!