Making watertight seal between flush pipe and WC ?

How to achieve the watertight seal needed between flush pipe and a horizontal blunt ended (no collar) WC inlet hole? All parts came with ceramic low flush WC. Have fitted the Internal Polythene Flush Pipe Connector onto the flush pipe. This plastic pipe is pushed home into connector's inner sleeve so that this sleeve runs up inside the pipe. It is a tight fitting. All looking good so far. The connector has a larger flange at one end and this end is pushed home into the aperture of the WC inlet butting against the ceramic inner chamber but there is an inner moulding that prevents a uniform fit so the flanges deform. Regretfully when tested the cistern, the force of the water under pressure as it enters the chamber of the three flush channels bore holes forces the water back past the connector and spills at underside of the flush pipe. If connector was removed from flush pipe and fitted into inlet the other way around then the larger outer flange would now sit flush outside but the direction of shaping of the polythene connector flanges does not stay fast that way round and pops out. Wonder, which is correct end to insert to WC and if the use of Fernox LS-X jointing compound will hold the water pressure and stop the spillage? No instructions or other parts with the W.C. Thanks for your help.