Spikes of blue flowers in leaf-axils. Upright plant growing on a stream bank.
No interactions present.
The ID Veronica anagallis-aquatica x catenata was suggested by "DigPeter" of the Flora of the British Isles group on Flickr yesterday.
I couldn't say, but pages 32-33 of the following BSBI News release may help.
Lat/Lng: 51.0506, -1.31872
OS grid ref: SU478281
Beside a fresh water channel, River Itchen watermeadows, St Cross, Winchester.