I've been flying round the Pacific rim in my RealAir Duke V2, with added GTN 750, and all was going well ... until I did a leg from UHHH to ZYCC. Now I rarely "fly" out east but it came as a bit of a shock during my approach to find (or not) that the airfield wasn't where the GTN 750 said it was. In fact in FSX:SE it was about 25 miles west !!!! Have the Chinese moved the airfield since FSX was first compiled or is there a big typo somewhere ?

Interesting.  I think you will have to find an up to date chart of ZYCC and cross reference the GPS co-ordinates with what is shown in FSX.  That will at least tell you if the problem is FSX, the GTN or ZYCC.