I don't think this is bipunctatus - it is a female, but the reddish colour at the base of the abdomen is a purple-coppery colour, which I think is a metallic reflection, rather than the orange-pigmented patches typical of bipunctatus. (The female of bipunctatus rarely lacks these patches, but it has orange coxae, so can still be distinguished.)
The occiput appears to have a fringe of pale hairs, thus placing it in the albibarbus/cuprarius/iridatus group. While the pale leg colour suggests albibarbus, the frons appears too broad (and it is a Mediterranean species, unlikely to occur on a North Wales cowpat ). So I think it is a pale-legged cuprarius or iridatus. (Very difficult to separate - need a specimen. )

You are right. I have changed the name to Sargus sp.

I agree it's not S. bipunctatus - why do you not think it's S. flavipes?