After a bit of research online it seemed as though a popular solution State-side is to use cat litter, mixed with a bit of water and then shake like hell!
Well i thought it cant hurt to give it a go. Lucky my mum has a cat and let me nick a tub full.
Anyway, i poured about half a cup of litter inside the bottle and topped it up with water until the litter was just covered. Replaced the cap, and sealed the overflow pipe with tape. Shook like crazy for about 10 minutes.... results:
Before:


After:



Id say its done a decent job! some minor staining of the plastic left over, but i think its engrained in the material! im happy with it now! Maybe it will help somebody else..