My wife & i were walking along the beach today and kept coming across what at first glance appeared to be pieces of polystyrene.
On closer inspection , they looked as if they might be clusters of eggs . They appeared yellow in colour, about the size of a Smartie, and when a cell was pinched ,liquid came forth !. The largest pieces were approx  225mm long by 75mm wide but were irregular in shape as had been rounded of by action of sea . We live in the north of Scotland,approx 10 miles from  John O' Groats. There were some big seas last week which threw up a lot of kelp  and a surprising amount of sea urchins.
Any thoughts as to what produced the eggs ?

These sound like Whelk egg cases. These snails lay clusters of eggs, often under rocks or in crevices along the beach, at or around the intertidal zone (where the sea covers at high tide, but does not cover at low tide).  When the snail larvae hatch (they develop all the way to little snails inside these 'cases') the egg cases often become dislodged and wash up onto the beach.  Sometimes you find unhatched snails in the cases still.

I hope this answers your question.  Obviously this is a little speculative, as I have not seen a picture of the material you are describing, but I think this is a good guess.  If you look on the internet, for whelk, or Dog whelk eggs you might be able to confirm my suspicions.

This link might help too:

http://www.dkimages.com/discover/Home/A … ies-4.html