(I know it isn't very clear... but there is an owl on the telephone lines... see?)
After some experimentation I discovered that a two tone silent whistle worked best. That is, a whistle that as well as an ultra-sonic note, also had an audible element. Most of these whistles are tune able to some degree. If you completely remove the locknut then it is possible to raise the audible note to a really high pitch just on the edge of your hearing. It is this combination of very high , and ultra-sonic that owls seem to like.
Trying to emulate the sounds of squeaking prey such as mice, voles etc would only bring the owls so far.
They would look around somewhat confused as to where the sound was coming from.
Luckily they didn't view me as prey even when I hid in the long grass or wore my furry hat.
I tuned the whistle down a tiny bit and copied a Tawny owl call.... Bingo!!!
Suddenly we were friends and the owl came flying over to me for a closer look.
The chap was quite happy for me to take photos and seemed interested in the raisins I had in my hand to feed the foxes. I wasn't sure if owls should eat raisins and although I checked my pockets, I hadn't got any mice on me.
I'll have to study the calls of Barn owls and see if they respond as well. Maybe Little Owls would be fun too as they come out in the day and so would be easier to photograph.
Must remember to put some Voles in my pocket for next time...
If you like your owls really big, then click on the picture and they will embiggen.