I have a kit very similar to that and no problems with the adapters fitting. I'd very carefully check the threads on the adapter, particularly the first turn to ensure there's no obvious defects or damage. Mesh the adapter threads with the threaded portion of a spark plug and hold it up to the light to see how accurate the thread pitch is. If that looks good it's possibly been machined slightly oversize, a set of verniers would tell you this if you have any.Thank you for getting back to me guys,
Here is the link to the kit i bought you should be able to zoom over the image to get a better look at them. i did use a guage from my tap and dye kit and it it sure is m10 1.0 but from looking at the plugs thread it looks angled slightly compared to the straight cut thread of the compression piece.
Petrol Engine Compression Tester Test Kit Gauge Car Motorcycle Garage Tool UK | eBay