This shows the male guarding a small web nest with the female within.
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I think it is a Clubiona but why do you suspect stagnatilis?
Well, I believe it's a stagnatilis because there was a striking match to the photo in the Country Life guide to spiders. It also mentioned that the Chelicerae are almost black in this species. Having said that, I'm new to the art of spider identification so there is lots of room for error. I also have been releasing them after taking the photos so I haven't dissected them to carry out any further checks.
Thanks for you help with this, by the way.
C. stagnatilis is found mainly in wet habitats e.g. marshland. Is that where you found it?
Personally I'd feel the need to examine the palps / epigynes of Clubionas to get them to species as many of them really are very similar. I wouldn't even feel confident about IDing ones with a pattern on the abdomen (e.g. corticalis) from a photo. Also, useful though the Country Life book is, some of the Clubiona photos are more helpful than others - e.g. I've examined many C. phragmitis but never seen one that colour - as the text says, they're light brown.
Lat/Lng: 53.09864, -1.39301
OS grid ref: SK407558
Alfreton back garden