  We present 850um imaging of the XMM-LSS field observed for 170 hours as part
of the James Clerk Maxwell Telescope SCUBA-2 Large eXtragalactic Survey
(S2LXS). S2LXS XMM-LSS maps an area of 9 square degrees, reaching a moderate
depth of 1-sigma ~ 4 mJy/beam. This is the largest contiguous area of
extragalactic sky mapped by JCMT at 850um to date. The wide area of the S2LXS
XMM-LSS survey allows us to probe the ultra-bright (S_850um > 15 mJy), yet rare
submillimetre population. We present the S2LXS XMM-LSS catalogue, which
comprises 40 sources detected at >5-sigma significance, with deboosted flux
densities in the range of 7 mJy to 48 mJy. We robustly measure the bright-end
of the 850um number counts at flux densities >7 mJy, reducing the Poisson
errors compared to existing measurements. The S2LXS XMM-LSS observed number
counts show the characteristic upturn at bright fluxes, expected to be
motivated by local sources of submillimetre emission and high-redshift strongly
lensed galaxies. We find that the observed 850um number counts are best
reproduced by model predictions that include either strong lensing or source
blending from a 15 arcsec beam, indicating that both may make an important
contribution to the observed over-abundance of bright single dish 850um
selected sources. We make the S2LXS XMM-LSS 850um map and >5-sigma catalogue
presented here publicly available.
