1976 Leyland Mini engine block coolant is blocked

Was doing routine radiator flush after engine overheated while on longer trip. Hasn’t overheated on around town trips. Radiator coolant was clear green with no lumps or other material coming from within the system. Went to flush the block and no coolant would come through the block drain. Put in radiator flush and ran with no change. Did another flush (leaving flush in for longer) with no change. Replaced thermostat and radiator cap (recommended by mechanic) with no change. Checked flow through heater and radiator with no problem. Car has only done 89000 klms and service records don’t show removal of head (ie unlikely that head gasket replaced incorrectly). Can insert 4mm diameter tool about 40mm into the block without obstruction but tool and plug are both very dry, no evidence of coolant. Water not leaking into sump nor oil in radiator. Pistons and cylinders don’t appear to have water in them. Water pump replaced at 78500 klms; radiator replaced at 79800klms. Absolutely certain I’m undoing the right plug.