Last one I did was a combination of
a large cooking apple, a pair of St.Michael underpants, and a piano
castor.

Errr.... I think caustic soda would be the least of your worries!!
Forgot to add that these items had all sucessfully navigated the u bend and
the soil pipe, and were lodged on a bend about 3ft beyond the inspection
chamber on their way to the main sewer. IME most blockages are nothing to do
with the WC itself. If your on shared drains, it could be halfway down the
street.