Practice Instrument Approach?
 

Is there any rule that forbids an a/c making a practice instrument approach followed by a go around at an airfield where the runway is not long enough to allow a landing?

If so where is it written?

There is an inturpretation on this in the US. I don't remember all of the details but I believe it was based on the rule which restricts minimum altitudes with the qualifier of "unless when necessary for taking off or landing". If the runway is too short for you to land then you have to comply with those minimum altitude regulations.