The water lobelia site is at Blea Tarn (the one in Langdale). It is perhaps best to access this site from Skelwith Bridge - from the A593 head south west and take the minor road leading to Little Langdale. Go through the village and take the right turn past a block of woodland. After about 1km, park in the National Trust car park and take the path west towards the Tarn.
If you walk around the tarn in an anti-clockwise direction, you will soon see stands of water lobelia in about 1 foot of water. Other species here include quillwort and red (alpine) pondweed. I remember finding awl-wort here a few years ago but I didn't find it on my visit last July.
If your're interested in dragonflies, there's downy emerald, golden-ringed dragonfly and keeled skimmer here too.