thanks to all above for your support, it is solved
Batty wrote:If it is a bad character in an ID then press CTRL-F and you can examine all the entity IDs.
Other than that, start removing entities one-by-one starting with the last one you placed, i.e. slowly undo the latest changes you made until preview works.
I looked at the .dat in notepad and it was huge, went on page after page, I couldn't see any mistake
so I realised it must be to do with a recent change, it turned out I had done a whole load of changes and just not hit PLAY so it was tripping on a change I made about 10 changes back
I saw a torch holder that was on remove was opening a nearby door, fine, that always worked long ago
I realised that a new approach route to the hidden area could result in the player coming up against a locked hidden door, so I added a new command to that torch holder
I added a connection make the torch holder open a hidden door many levels away but I FORGOT to change the drop down to Remove, so it showed INSERT
it was THIS that was causing that error
I simply changed the 2nd command to onRemove nad it played fine, no error
THANK YOU TO ALL ABOVE FOR YOUR SUPPORT
I AM BACK IN THE GAME!!