Harvestmen are arachnids, related to spiders. They have eight legs and two palps at the front, which with some species can look like an additional pair of legs.
Many can be found in leaf litter but also wondering around on walls or trees. If you go out at night with a torch, you are likely to see many more species hunting.
The current species list for Guernsey is:
I have added some photographs of those species I have seen which may help with identification.
Dicranopalpus larvatus
Dicranopalpus ramosus
Histricostoma
argenteolunulatum
Leiobunum blackwalli
Leiobunum rotundum
Mitotoma chrysomelas
Nemastoma bimaculatum
Opilio saxatilis
Paroligolphus agrestis
Phalangium opilio
Scotolemon doriae