19 heater not working

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My '93 1.4RT 19's heater's about as much use as a candle in a hurricane. It's OK if the outside air temps above 6 degrees, but on frosty mornings it is hopeless. Added to which the engine takes forever to warm up. It's had numerous thermostat changes, I've flushed the heater umpteen times, last night circulating a solution of washing machine descaler with pump then pressure washer-flushing, all to no avail.
I was told when asking on another forum that these cars cooling systems are tricky to bleed and you need to get the front end 4 feet in the air to do it properly. SInce I haven't done this I thought it might be airlocked. Another mate said try rigging a sealed hose into the header tank and connecting it to an old lemonade bottle half-full of coolant, hung up suitably high, and run the engine so that any air bubbles can work their way out.
Has anyoen else had this problem? DO these engines/cars suffer from silting in the cooling systems? Is there a shortcut when changing the matrix or is it dashboard out and that's that? Has anyone shot the twit at Renault who designed a heater that was absurdly easy to get at then made it with pipes that go out through the bulkhead, effectively preventing easy removal?
Any advice/tips/hints gratefully received.