Problem solved.
Thanks everyone for your help, and your patience with me especially.
The problem in the end WAS the impeller. I removed the pump again today, and re checked the impeller for debris. What I didn't realise previously, was that the impeller actually unscrews from the pump motor, and can be cleaned out of the pump. I wasn't properly checking the impeller vanes for blockages. Turns out there was some form of gum nuts, stuck into the vanes and had basically blocked the enitre flow.
After proper cleaning, the filter is functioning at about 80 kpa, which is a great improvment on 0!!
Thanks again for all your help, and I hope this may help any new owners out there understanding the basics of there pump, so they dont get into the same situation I did.
Swimminng here I come
