OH...what a piece. And I like Britten.
So this is an example of continuous variation on a theme in the tenor (sung by the tenor rather) intead of in the bass. It's nine iterations of six measures each in gm, cadencing melodically always strongly on the g -- made an IAC by the fact that the orchestra never has a solid g in the bass (though it's atonal, so...). The last variation ends without anything under it, so you could consider that a PAC. And at the third measure of the repeated material, there is definitely something of a weaker cadence (like an IAC). So you could split it into two contrasting, symmetric phrases that form a period.
One obvious variation I'd like to point out is the text, as the words change for every repetition, only the last verse is the same as the first.
1. By itself, end elides into the repeat, as low strings begin.
2. A slight rhythmic variation - half note reduced to dotted quarter, and an eighth pickup note takes up the difference. Strings are in unison and low. But break at the second part of the variation, where a dotted eighth and sixteenth are inverted.
3. half notes restored. Higher strings start to climb out of the texture. Constrasting rhythms in the strings and triplets against sixteenths. Odd leaps and melodic jabs in the bass.
4. one half note kept, the second not. Violins finally begin to raise into their register. They attack epeated notes of changing rhythmic divisions before leaping in a sort of gesture. Notes change at teh beginning of the second phrase and rhythm (triplets and repeated b), and then more triplets two beats away.
5. First half note not kept, second kept. A run down the register in the strings with grace notes. Upper strings build almost chords and low voice rumbles in sixteenths running in neighbor patterns and scales. For the second phrase, trills in the bass add tension.
6. Horns! at the beginning of second phrase, rhythm changed again -- 4 sixteenth notes instead of the dotted eighth sixteenth.
7. Horns die down and out. Texture thins and high violins slink around. Again rhythm change in same place, but here two eighth notes.
8. Lots of static intervals repeated in different subdivisions. Same rhythmic difference as before.
9. high trills signal the dying off of violin, bass runs down to the cellar agitates, then hits three last one voice attacks and goes out. Soloist ends on his own and does not make the octave leap up to the high g, but remains on the low g.