High-redshift , luminous , dusty star forming galaxies ( DSFGs ) constrain the extremity of galaxy formation theories .
The most extreme are discovered through follow-up on candidates in large area surveys .
Here we present 850 \mu m SCUBA-2 follow-up observations of 188 red DSFG candidates from the Herschel Multi-tiered Extragalactic Survey ( HerMES ) Large Mode Survey , covering 274 deg ^ { 2 } .
We detected 87 per cent with a signal-to-noise ratio > 3 at 850 \mu m. We introduce a new method for incorporating the confusion noise in our spectral energy distribution fitting by sampling correlated flux density fluctuations from a confusion limited map .
The new 850 \mu m data provide a better constraint on the photometric redshifts of the candidates , with photometric redshift errors decreasing from \sigma _ { z } / ( 1 + z ) \approx 0.21 to 0.15 .
Comparison spectroscopic redshifts also found little bias ( \langle ( z - z _ { spec } ) / ( 1 + z _ { spec } ) \rangle = 0.08 ) .
The mean photometric redshift is found to be 3.6 with a dispersion of 0.4 and we identify 21 DSFGs with a high probability of lying at z > 4 .
After simulating our selection effects we find number counts are consistent with phenomenological galaxy evolution models .
There is a statistically significant excess of WISE-1 and SDSS sources near our red galaxies , giving a strong indication that lensing may explain some of the apparently extreme objects .
Nevertheless , our sample should include examples of galaxies with the highest star formation rates in the Universe ( \gg 10 ^ { 3 } M _ { \odot } yr ^ { -1 } ) .