Only a few mm across. It was on the rose but I transferred it temporarily to the (dry) bird bath for part of the photo shoot.
No interactions present.
I'm not at all sure about this spider, and regret not having had a closer look for a red patch at its "tail" but after looking at lots of pictures on-line I found this site
http://dipode-vie.net/Arachnides/Araneidae/Araniella/sp.html, and especially this picture of an immature female Araniella http://dipode-vie.net/Arachnides/Araneidae/Araniella/BestOf/Araniella,%2... which tipped me towards Araniella.
I think you're right - the only other spider I wondered about was an immature Araneus quadratus, but the arrangement of the black dots on the abdomen looks more like Araniella.
Lat/Lng: 52.061168, 1.159024
OS grid ref: TM166450