  Deep sub-millimetre observations using SCUBA are presented of the central
regions of four high redshift clusters which have been extensively studied
optically: CL0023+0423 (z=0.84), J0848+4453 (z=1.27), CL1604+4304 (z=0.90) and
CL1604+4321 (z=0.92). 10 sub-millimetre sources are securely detected towards
these four clusters at 850 microns, with two further tenuous detections; the
raw 850 micron source counts exceed those determined from blank-field surveys
by a factor of 3-4. In particular, towards CL1604+4304, 6 sources are detected
with S(850) > 4 mJy making this the richest sub-mm field discovered to date.
Corrections for gravitational lensing by these high redshift clusters reduce
these excess sources counts, but are unlikely to account for more than about
half of the excess, with the remainder presumably directly associated with
cluster galaxies. The 450 to 850 micron flux density ratios of the detected
sources are systematically higher (at a significance level > 98%) than those
determined for blank-field selected sources, consistent with them being at the
cluster redshifts. If subsequent identifications confirm cluster membership,
these results will demonstrate that the optical Butcher-Oemler effect is also
observed at sub-mm wavelengths.
