Hi Graham,
I interestingly enough have spent most of the day reading problematic boards all related to the same Samsung side by side American style fridge freezer - well not so interesting but fed up trying to resolve issues upon the RS21DCNS, both my parents and myself have the same unit which we purchased two & half years ago... guess what, just after the waranty ran out we both started having similar breakdowns/ problems/ issues.
Most of the time it seems to relate back to defrosting the machine - try this first, the water pipes are likely to be frozen... turn it off for 24 hours.
Samsung will of course deny that there any any issues but if you take the time and search google for relaste details, the consumer list is overwhelming...
Steven - London, January 2008