Finally resolved!
Turns out it was the short linkage that isn't included with the short shift kits.
I bought two 8mm rht rosejoints from ebay for around £5 and a length of 8mm threaded rod for about £2.
Took the old linkage off, and used it to measure the new rosejointed linkage against. I used the original two bolts from the old linkage and bolted it in place. This is tricky when the gearbox is on the car, but still achievable.
The mistake I made was the bolt that joins the short shift and the connecting rod (part 19), wasn't located in the dimple properly. At some point between fitting the short shift and the linkage (above) it has slipped, causing play. Once I realised and tightened it up all was well!
I now have about one inch of left-right movement. I guess this is down to manufacturing tolerances and wear from withing the gearbox its self.