No seedlings at all

HIya

I'm not quite sure from your post - but if you sowed the seeds directly into manure that may have been a bit rich for them.  You could try buying a bag of bog standard compost - using that to fill the drills and sowing the seed into the compost to give them a head start before the roots hit the manure.   It's probably not too late to try again.  (Having said that I've double sowed in the past as I was sure things had died and weren't going to come up - and they did - which led to a crowded bed...). 

In that case - maybe you were just a bit early.  I'm not sure that the straw would have been a problem if it was under 2 bags of compost.  Try again....

Have you been out at night to check if there are lots of slugs. Maybe they are all just getting eaten? I go out with a head torch and flick all the slugs I can find into a container and relocate them away from my garden/allotment

I woudl suspect slugs too.  There were things I sowed up the allotment that almost totally failed, and I know they were germinating, they just kept disappearing again.  Too small for the rabbits to bother with.

I think slugs have waited for germination & chomped the lot off.