So I tried shimming the magazine which didn't solve it, but found that if I released the magazine catch and sat the magazine a mm lower in the magwell by hand it fed and cycled fine so I ended up solving it by literally removing half a mm from the top of the magazine catch as it was rounded whereas the Cyma magazine release catch is flat and it has solved it and the original magazine (tactical maraca hicap) still fits and feeds without issue... Although it will never be used!