So, got it sorted.
Firstly i found the hose leading to the washers and used my electric car tyre pump to blow air through them and that came out fine, pushing out the last little bit of moisture.
Then i tried the opposite way through the tank. That bubbled up fine so initially i thought it was clear. however i carried on doing that and after a bit and some green slime came up so i filled it back up with the garden hose under pressure and it pulled some more slime up and out. Then the air pressure again, then the hose again etc.
I did that for about 20 minutes and gradually no more slime was coming up i topped the water back up and tried it and it's working brilliantly.