Drained my coolant today, connected hose up to heater pipes at the bulkhead and flushed through both ways a couple of times and did get some shite out but it was only when I connected the airline up to it to blow the water out (in readiness for the coolant) did I get a mega amount of crap out so I flushed with water again then blasted with air again etc etc. Put back together with new thermostat and only filled via the rad, left ticking over with heater blowing max and full .....didn't seem to be filling/bleeding up properly, 15 mins later heater still blowing cold so I topped the expansion bottle up and took it for a blast up to 100 or so then all of a sudden......hey presto.....its blowing hot ....YAY
Lets hope it stays this way.