It is indeed a woodlouse spider and while it has a painful bite it is not poisonous. They do get in compost from time to time especially if their are a lot of little critters in their. If your adding meat to your compost I would not. Especially in summer.
The woodlouse spider will feed on other available small insects if the wood lice is not available.
Good luck with that Izzy. Do you think that it came in on the compost or found its way in there? If it was an import, you could always try buying another bag of compost at the same place and see if there are any more stowaways?
The BBC says they can be found under logs, do you have a log pile?
I generally like spiders, but this is a very strange looking one.
The compost it was found in has a checkered history. It was a bag of good quality rose compost, can't remember the name at the moment, but it might have been John Innes.
It belonged to a small local commercial growing company who sadly went bust, and told me to help myself to the remains of plants, compost and pots from their industrial greenhouse site, 3 years ago, or it might have been 4 years.
These bags of compost had been stored for a long time, and were already bursting when I got them.
They then lay in Dad's garden for a year or so, before this one got moved into the greenhouse and opened.
It has lain there ever since, unused but open, allowing the soil at the very top to dry out, but not further down, strangely enough.Maybe there is a rainwater drip overhead.
As this seems to be the first woodlouse spider found in this neck of the woods (I'm in Kintyre which is almost an island, so when wildlife finds its way here, it has almost certainly been imported), the question is did it come with the compost (as a baby perhaps), or did the last growers import it with plants they were growing (even from England).
This spider was big and fat, so probably a female. Has she had babies? Was she pregnant?
There are bound to be more around anyway.
Need to watch out for them, and not handle compost with my bare hands.
And no, no logs (but plenty trees), but I just found a toad in the garden too, sheltering under a black plastic sheet covering yet more bags of compost, and I'm told they like logs too.