Ok fond where the heater pipes go through and disconnect them then with a garden hose try flushing one way then the other.
At least you are sorted and got it done for a good price.When i ring halfords autos they said £37 including coolant. Next morning I dropped my car they said u can take it later on today but when i went there to pik my car they said £109 and i was shocked. Brand manager told me that the guy over the fone told u wrong price but i said was that my fault??? Anyways after a long argument he said ok pay £77. Manager was so rude and was not accepting his mistake. I paid £77 and still happy atleast my biggg problem is fixed. I was upset that why they told me wrong price even when i confirmed 3 times and launched a complaint online.