Hi HillSalt
Yes its been a very cold winter and the ground temperatures are far lower than they would usually be at this time of year.
I would recommend you aplly as much slurry to your lawn as you can at this stage. If you are not a farmer you could approach one I'm sure they would allow you to take some.
Slurry is excellent for a number of reasons it will increase the temperature of the ground you apply it to plus it will increase the growth of your lawn