Another source says 1926. Then there's this one:

We have been sent the words to a song, sung by Mrs Louise Wilkinson, now of Frinton & District Trefoil Guild, to the tune of Men of Harlech when she was a Girl Guide in Sheffield in 1928:

THE SONG OF THE ANCIENT BRITONS C.1927.

What's the use of shirts of cotton
Studs that always get forgotten,
These affairs are simply rotten,
Better far is Woad.

Woad's the stuff to show men;
Woad to scare your foemen:
Boil it to a brilliant blue
And rub it on your back or your abdomen.

Ancient Britons never hit on
Anything more good than Woad to fit on.
Front or back or where you sit on -
Not a nest for fleas.

Romans came across the channel
All wrapped up in tin and flannel:
Half a pint of Woad per man'll
Dress us more than these.

Romans keep your armour;
Saxons your Pyjama's.
Hairy coats were made for goats,
Gorillas, yaks, Retriever dogs and Llamas.

Tramp up Snowdon with your Woad on:
Never care if you get rained on blowed on.
Never need a button sewed on -
W! O! A! D! Woooad!!!.

Anon.