Thanks to Glyn, who sent in this picture of mystery eggs found in his garden, in a wet, cool spot under a concrete slab. They are in fact snail or slug eggs – I have no idea if it's possible to tell the difference easily. Presumably fairly big ones though.
Slug eggs. The cue is that they are almost transparent. The calcium carbonate crystals in snails eggs – used to build their shells – makes them white. The location is also more consistent with a slug, as large garden snails dig in soil to lay eggs.
Nice! Great info – thanks.