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It seems odd for Myosotis scorpioides to grow into a mat like this. I wondered if it might be the invasive Ludwigia grandiflora, but I have no experience of it. It will become obvious when it flowers.
I thought I found a single Ludwigia a while back (a few miles over the Staffordshire border), but it was disposed of (the whole edge of the pond was sprayed, even though it was supposedly a wildlife area) before it flowered, and John wasn't positively convinced of its identity.
Looking at photographs the shape of the leaves of Ludwigia grandiflora varies considerably, from ovate to lanceolate, but they tend to be flat, with noticeable pinnate venation, and have a somewhat different habit.
This observation shows (I think) a plant with both last years shoots, and new shoots forming. But Ludwigia grandiflora is supposed to be deciduous.
I agree that the photograph doesn't look right for Myosotis scorpioides, but I don't have an alternative suggestion.
Thanks cicuta58 & lavateraguy for the comments. It's not my pond, I'm afraid, and I won't be there when they flower - but I'll try and get a friend to email a pic when it does. However, having had a good look around on the 'net for Ludwigia, I'm dubious that that's the answer. Will be patient and see how it goes!
The plants on said pond are all apparently IoM-native, though that doesn't preclude a non-native sneaking in.
Flickr pics: http://www.flickr.com/photos/72243477@N00/
Lat/Lng: 54.235, -4.4071
OS grid ref: SC432848