Thanks all. Seems this weekend is spoken-for, but I'll try some of those thoughts and report back when I've made some further progress. @just pumps no, she doesn't use those things. Don't know if it makes any difference, by the way, but we have a septic tank. All other drains in the house seem to be running freely, although this one is the one furthest away from the tank.
Toilet frequently getting blocked
- Thread starter Avocet
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Be sceptic of the septic 
Septic tank shouldnt make a difference, unless the tank outfall isnt working properly, and the pipework back to the house is full of backed up effluent. If that's clear and the problem seems to be lying between the manhole and the WC, then I'd say you've a partial recurring blockage. Rod it and see what you can pull back. (Dont lose the rods down the pipe!)