After lots of googleing and a few calls i came to the conclusion that the options i had fond were too expensive at £50+ for an adjuster. I did find one from Harris Performance (on ebay) it was a bit sort but at £25 a better price so i thought i would buy it and then work out what to do. So when it arrived it fitted nicely but was about 15mm to short when fully opened, so i bought a female rose joint end for £2 from ebay and a bit of threaded bar and now it fits perfect.

Opel Manta alternator adjustor bracket

Opel Manta alternator adjustor bracket