You should be able to check that to a reasonable degree of accuracy by going to a local spot height as marked on the Ordnance Survey map and comparing what your GPS says, taking a few readings to average it.
This is a useful link for looking at the large-scale maps http://www.geograph.org.uk/showmap.php?gridref=SN580810
Or this shows heights to 0.1 metres in your area: http://map.ceredigion.gov.uk/connect/?mapcfg=PLANNING_APPLICATIONS
Many thanks. I think I was close enough. I'll PM you a photo so that you can work out the height of Ladder Wall above the water..
Here is the tide: http://www.ukho.gov.uk/easytide/easytide/ShowPrediction.aspx?PortID=0488&PredictionLength=1