Sweetcorn seedlings dying off

The only advice I can offer is do not over water, the soil should be damp not wet.

As an afterthought, is the compost reliable, I remember lots of problems last year with Homebase compost

It could be that they didn't like the peat-free Aldi compost they were in. There's no pesticide contamination, as far as I can tell, since most other things have grown in it. I'll try it in some other stuff that we have at the house this time to see if that'll help. Thanks.