I was able yesterday evening to hear a recording of this bird and I have to say it sounds a lot like moltonii to me, though it would be good to get a better recording. My only concern is that I would like to make sure this is not just a rattle introducing the song, however, from my notes on Corsican birds, moltonii seem to do this call as a prelude before including it into the song, and from what I can hear there seems to be the same thing happening here. Mike Pennington on Unst has heard it calling like a moltonii when not singing, so that is very positive.
There is a recording of the call of moltonii (NOT the Shetland bird) at this link (scroll down) http://www.ebnitalia.it/QB/QB012/sterpazzoline.htm. Albistriata sometimes have a 'trrt' repeated two or three times, and can have a slight 'rattle' in the song, but I have never heard them call like this, as I have never heard cantillans call anything other than 'tec'. From reading Shirihai et al (Sylvia Warblers), they mention a general 'fast series of ho.... or gutteral notes,..... 'tr-tr-tr....'', commonly used when alarmed near the nest.'. It is hoped that this is not this call, though I have no experience of it, despite being near nests of cantillans and albistriata.
There are images here
I have been looking at these images recently, and as ever trying to fit them to a subspecies, and HH's images have really struck me as showing a tone and colour I would associate with moltonii also - a pale dusty pink, with limited orange hue, if any (though I note that other images seem to show this orange). The upperparts of moltonii seem to be nice clean powdery blue-grey (which is on this bird also?) I would certainly find it hard to believe that the images show an albistriata. A really good field assessment of the colour and the relative strength of the throat compared with the underparts, and th colour of the upperparts, would be helpful.
I look forward to more information and input from Andrea in Italy (I can send a recording if you would like it, Andrea).