However Achaearanea really need adults under a microscope to identify with reliability and even then they are rather difficult to separate. Whilst it might seem reasonable to assume from the location and position in the habitat that this must be A. riparia, there are other species in Europe which could potentially be present in Britain and for which microscopical examination is needed. We have already had a number of spiders new to Britain recently which would not have been recognised without voucher specimen/s and microscopical examination.
Achaearanea riparia is a scarce spider, so it would be very good to gain definite confirmation of your spider. Can you provide information on the location with grid reference etc.