An email and conversation with Brian Small has revealed that this bird is in fact the rarer subspecies limosa. Cornwall regularly attracts the islandica (Iceland) race, some of which winter in Cornwall, mainly on the Camel and Fal estuaries. This subspecies breed mostly in Iceland.
The limosa race winters in west Africa and breeds in Holland and further east towards central Asia. A few pairs breed in East Anglia and there are some records from Slimbridge. Thus, this race is indeed rare in the west country. As far as I'm aware, the only previous documented record for this subspecies in Cornwall was a party of seven at Maer Lake, 3rd April 1995. (GPS).
The islandica race tends to have a smaller bill, shorter legs and a more burnt-rufous colouration (extending to the belly) than limosa. The bird in question below is long-legged, long-necked, long-billed and the eye appears set back from the bill, giving the appearance of a large head and open lores. The two races' moult patterns also differ. The bird below is unmoulted in the upper wing coverts and has mainly plain grey coloured tertials and wings with limited new patterning in the mantle.
|limosa - note the pale orange hues stop abruptly on the white belly. The barring below is sparse.
|limosa - the plain grey tertials and wing coverts contrast with limited patterning on the mantle.
|limosa - compare the head shape and bill length with the islandica below.
|islandica for comparison - St Clement, Truro. April 2012. Note the extensive patterning on the mantle.
|islandica for comparison - St Clement, Truro. April 2012. Note the dark orange colour extends to the belly.