Advice on a thread size and fitting for a Celestron binocular tripod mount
The standard thread for these is 1/4 - 20 UNC.  The 20 means 20 threads per inch - you could try measuring that. The same size fastener should be what's on the Opticron, so it should "just work".  It's possible that there is something in the hole in the binocular that's blocking the thread.

However (and if you are not the first owner),  someone might have tried to use the wrong fastener and damaged the thread.  Threads can be recut with the right tool.