This is quite a large spider, its head and body alone are over 15mm (more than half an inch). What is it and what's it doing?

It is a Costa Rican spider, Ararchnida circusi, which traps insects by rolling over them over while balancing on its sticky ball of silk

It is Brachypelma Icelandi, found in Scotland, Scandanavia and Iceland, the so-called 'Snowball spider', which lauches a sticky ball of its silk at its victims

It is a British spider, Pisaura mirabilis, the Nursery web spider, which carries its babies around in a silk cocoon