In reply to Graeme G:
While a sunset is not possible(and I reckon even without the clocks changing it would have been very difficult, my preference is to be at a sunset site at least 30mins before), but you say you are there for a day.
Therefore is a sunrise on the following day a possibility?
A fair bit of the east side of South Uist is accessible only by walking but some where like the north end of the causeway connection South Uist to Benbecula, or Port Pheadair on the south east of Benbecula is more easily accessible. If you put Port Pheadair into Google Maps you see where it is.
Dave