The Mayday edition of the Padstow Echo has just arrived and it contains an interesting article by John Buckingham.
Apparently the Day Song back in the 1890s was much more like the Halantow and at one point has:

With Hal-an-tow, and jolly rumble O.
For summer is acome, and winter is ago,
And in every land O, the land that ere we go.

It appeared in print in 1895 as a broadsheet by Harding the stationer, then the same words were published by Williams and Son in 1903 and by Thurstan Peter in 1913 in his book "The Hobby Horse".
Sometime between then and 1950 the Day Song was abbreviated to the few lines now sung when the Oss dies, with no mention of Halantow.
The tune sung today doesn't appear to fit to the old words.

The tune given in Malcolm's first link above is the Furry dance tune but was the Halantow originally sung to the same tune?

Was that song widely known throughout Cornwall but gradually lost to all but Helston and Padstow?
Was it originally a Helston song adopted in Padstow?
Or was it part of the Padstow festivities and adopted by Helston?

Padstow was a shipbuilding, sailing community up until the early 20th C (unless you count fishing which is still important there today) but Helston is a little further from a port, if Winfree is correct and it was a sea shanty.
The Padstow version has 'French Dogs' eating the grey goose feather, not Spaniards, and there is a legend that a French privateer was headed for Padstow but was frightened off by seeing the Oss dancing up on a hill. They thought it was the devil and the rest of the dancers in red were his imps. Some put the date for this in the 18thC, others say "in ancient times" but the dancers in red were supposedly women in their red capes, because the men were away fighting somewhere else.
One verse of the current May song could have an answer to that:

Where are the young men that here now should dance?
For summer is acome unto day,
Well, some they are in England and some they are in France,
In the merry morning of May.

At Helston there is a black-clothed devil and red 'imps' who are chased away by St Michael.
So perhaps there is a link between the festivities at Helston and Padstow - or perhaps they're the only remaining examples of something that was once widespread throughout Cornwall/England/the world??
Robyn