Hi reubud .
smokeymacpot , sounds like your seedlings are damping off . This rots the base of the stem at compost level . If this is the problem go to any garden centre at get some Damping Off solution , it has Copper Sulphate (i think) which will cure the problem . Best one i have found is Cheshunts compound , blue powder . Make some in a 4pt poly bottle & follow directions but more importantly ensure you water the compost with it before you place seed/seedling in pot .
Stay safe all . BGB
i couldnt see rot, i'll try to explain better.. the seeds that do grow and break the surface seem to have one of the seedleafs dry out and when the dryness reaches the stem it wilts and the whole top dies, the stem and root is still full of water and crunchy etc. i have also seen damage further down the stem like its been scraped, and i was thinking maybe perlite scratched it as it grew past it. but i think that is not related to the seedleaf problem.
its only leb27 that has this problem, they are very hard to start :/