What is this - nest, eggs, fungus?
Updated Answer:

Why it's not the egg sac of Argiope aurantia:

Upon knowing your location to UK, we can rule out quite a few spiders, especially the Argiope aurantia since it is endemic to Northern and Central America.  The spider's egg sacs are also noticeably smaller than "a hen's egg" as this post describes.

Why it's not a gall:

We can rule this out since it appears to be located on a wooden fence post.  Galls require living plant matter to grow.  

Why it's not a wasp nest:

There are not wasps that produce threadlike materials. Wasp nests are made from fibers of dead wood and plant stems mixed with saliva of the wasps, and no webbing or threadlike material.  In the picture, we can clearly see some white 'webbing' between the gap of the fence posts directly underneath the mass.

So I really think it is an...

Aethalium of a slime mold:

Given the location, described area, spreading pattern in the picture, and apparent spongy texture of the mass, this looks to be the aethalium (or fruiting body) of a slime mold.  My guess would be in the genus Reticularia or Enteridium, which are typically seen in its reproductive phase as a white 'swelling' on standing dead trees in the spring, or on large pieces of fallen wood.  For example, Enteridium lycoperdon are common throughout the UK and are commonly associated with pine trees, the most common wood used to make fences in the UK.



*This post was edited from my original response.  Please refer to the edit history concerning the previous answer.