Growing in clusters out of the concrete cracks.
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Thanks for the ID, sorry about the delay in responding. I am very much a novice, so can you tell me whether there is something in the photograph that makes you immediately recognise the plant as a Garden Grape-hyacinth, or are you basing your ID more on the known geographic history of the plant, such as reported sightings? I notice NBN Gateway records the Garden Grape-hyacinth as prevalent in South Wales, while the Grape Hyacinth has reported sightings to the east, west and north of us, even south if you cross the water. Genuinely interested.
As one who agrees with Cicuta, I'll take this on.
Muscari neglectum, amongst other characters, differs in its distinctly darker flowers. Your photographs are typical M. armeniacum.
There is a history of misidentification of grape hyacinths and while M. neglectum does escape outside its native range (the East Anglian Breckland), it rarely does so. The majority of such records undoubtedly really refer to M. armeniacum.
Do not regard the NBN has having a lot of reliability here. Records are submitted by many disparate groups, many knowledgeable and authoritive, but some not, and the NBN has no error checking. Local Records Centres and similar organisations do not employ whole teams of experts, and someone good on, say, birds, may not have the knowledge to recognise unlikely identifications in other groups. With any critical species, it is important to check where records have come from. Records submitted by the BSBI should be for the most part reliable, theoretically having been sifted by vice-county recorders, but I wouldn't have much trust in other data sets.
[minor edit, 7.50pm, ref. NBN]
and have just uploaded photos for comparison - see http://www.ispot.org.uk/node/178189. The lower flowers are much darker than in your photos. (I'm assuming this isn't some special Greek form - I don't know how much variation there can be in colour.)
Thanks for the information and advice.
Lat/Lng: 51.718261846489, -3.8549888134003
OS grid ref: SN719037
A typical garden with grass, trees, bushes and paths. Road was built in the early twentieth century. Semi-rural location. Surrounding woodland extends to farmland. History of industry in the area.