Internal Overflow Cistern

I've just fitted a concealed cistern for a back to wall pan, the water is not yet connected. If the siphon doesn't have an internal overflow is there any way of retro-fitting one without using a turndish or external connection to the flush pipe? Could I drill a hole in the siphon at the appropriate level or would this screw up the flush?

Thanks,

Buy a new syphon with an internal overflow.

Hi John. The overflow relates to the filling of the cistern not the syphoning. This is in case the filling device leaks and the excess water needs a escape route. This normally a pipe leading outside, but there are some filling methods where the overflow leads to into the pan.
The syphon is basically an inverted 'u' tube and water has to be forced up the tube and over the top of the 'U' to get the syphoning started.
Drilling a hole in the syphon would prevent the syphoning from functioning

Thanks for your advice. I fitted the supply pipe yesterday, and filled the cistern (keeping a bucket handy and my fingers crossed). It turns out that it does have an internal overflow.

Cheers,