Hi Wellies,
According to Charles, the nutrients in ripe compost is NOT water soluble, and thus does not get washed out by rain. I use black plastic cover when establishing new beds, on top of the compost mulch and pasture, to block out all light and rain (i’m not really sure why Charles recommend keeping out rain when establishing new beds, but is expect it is beneficial for the decomposition of the compost into readily absorbable nutrients and soil micro life and structure in general (fungi etc)
Regards
Erik