Hi Meadow Pipit
Your mystery warbler is a reed warbler.
The other confusion species would be sedge but the back would be more heavily streaked back and a prominent pale eyestripe. At this time of the year we often seen reed warblers (and bearded tits) feeding on insects along the reed edges.
Hope that helps
Paul
Thanks Paul. It had crossed my mind that it might be the somewhat rarer Marsh Warbler. Thanks for the expert input.