I think these are probably snail eggs.
Yes, they are usually laid as individuals in a batch, but there are reports of strings of snail eggs, eg.
I appreciate the response. I have put the eggs in an old fish tank and will let them hatch to check what they are, but not totally convinced they are snail eggs. They appear to be bigger than any snail eggs I have seen before and the chain of eggs was at least 7" long which seems a lot for a single snail bearing in mind the size of the eggs.
Thanks for the info; I suspect you are correct and they are slug eggs, but suprised as I havn't seen any large slugs in the garden for a long time! Will confirm what they are once the eggs hatch.