The book says your boiler, running at max efficiency, will produce 2.9 litres of condensate per hour
Even less so when mixed with 6 or 9 litres of water.
After 3 hours between toilet flushes the boiler will have produced 8.7 litres of condensate so the water in the 6 litre cistern will be quite acidic. Any metal parts will be subject to corrosion.
Canned tomatoes pH range is 3.5 to 4.7.
Tomato juice pH range is 4.1 to 4.6.
Likening the acidic condensate from boilers to be similar to tomato and orange juice is a little misleading.
The pH of condensate from a boiler is typically 2.9 to 4