I had a brain wave! I had noticed that some of the junior teachers were inserting an audio file for spelling words. This got me thinking. I could do the same but instead record my instructions so that my boys new exactly what to do. (see below)
This has made a huge difference. Suddenly my instructions are rewindable, pauseable and much clearer. The boys aren't interrupting me anywhere near the same and I have noticed the quality of their work has improved also. I have also decided to keep my written instructions so that the boys are able to read along with the audio instructions.