  We present the cross-identification and source photometry techniques used to
process Herschel SPIRE imaging taken as part of the Herschel Multi-Tiered
Extragalactic Survey (HerMES). Cross-identifications are performed in map-space
so as to minimise source blending effects. We make use of a combination of
linear inversion and model selection techniques to produce reliable
cross-identification catalogues based on Spitzer MIPS 24 micron source
positions. Testing on simulations and real Herschel observations show that this
approach gives robust results for even the faintest sources S250~10 mJy. We
apply our new technique to HerMES SPIRE observations taken as part of the
science demostration phase of Herschel. For our real SPIRE observations we show
that, for bright unconfused sources, our flux density estimates are in good
agreement with those produced via more traditional point source detection
methods (SussExtractor; Savage & Oliver et al. 2006) by Smith et al. 2010. When
compared to the measured number density of sources in the SPIRE bands, we show
that our method allows the recovery of a larger fraction of faint sources than
these traditional methods. However this completeness is heavily dependant on
the relative depth of the existing 24 micron catalogues and SPIRE imaging.
Using our deepest multi-wavelength dataset in GOODS-N, we estimate that the use
of shallow 24 micron in our other fields introduces an incompleteness at faint
levels of between 20-40 per cent at 250 micron.
