[MikeHardman]

I bought this lavender a couple of years ago, and it has grown very well. But I am disappointed by the paucity of flowers and the large number of flies that seem to live amongst its foliage (brush by it and they form a temporary swarm before settling back). The flies are mainly quite small and blackish, but greenbottle flies (larger) are also attracted to it.
I'm sure it is not L. dentata because the leaves are not at all dentate. I hope you can see enough of the character of the leaves and the plant as a whole from these photos. I also include a photo showing another instance of this lavender, with an ordinary L. angustifolia a little beyond for comparison - the latter being smaller, much more floriferous, and fly-free.

[oron peri]

Mike
The large specimen is Lavandula x allardii, the small one is Lavandula x intermedia 'Grosso'.

[MikeHardman]

Thank you, Oron, for the names and for correcting my naive and wrong assumption about L. angustifolia.

I note a good description of Lavandula x intermedia 'Grosso' here -
http://www.herbcompanion.com/herb-profile/herb-to-know-lavender-grosso.aspx

[Alisdair]

There is some taxonomic uncertainty about Lavandula x allardii. Currently Kew regard it as a synonym of Lavandula × heterophylla, though without a great deal of confidence.

[Marilyn]

My two penn'orth:

I think it is not Lavandula x heterophylla/ allardii (or even "Goodwin Creek Grey"/ "Silver Form"/ "Americana" or any of the many synonyms for this confused cultivar): as Mike says in the original post, the leaves are not at all dentate. The flowers (and their shyness in appearing) are absolutely correct, but I would suggest this is another branch of the cultivar. It is nothing like either L. latifolia or L. dentata, the presumed parents of the cultivar, but neither is it quite like the plants I have seen of L. x heterophylla. Curiouser and curiouser! I can send this to Joan Head, who was the plant collection holder for lavender in the UK, and I will let you know her verdict.

One other point about lavenders that I only learned recently (also via Joan) is that angustifolia types (and presumably, therefore, their cultivars) are natives of northern Europe and need a bit of short sharp frost to set flowers well. I had observed this in my garden without knowing the cause - very few flowers on the L. angustifolias most years. This year, after some unusually cold weather and frost, they are putting on a proper display again. It has not helped the L. x heterophylla types, though... so I can only offer my empathetic commiserations there, Mike.

[MikeHardman]

Thank you Alisdair and Marilyn (yes, please do ask Joan)

[Marilyn]

Somehow my message to Joan went astray but has now been found; she will reply directly here when she has news.

[Lydia]

(From Lydia aka Joan) I would think that the larger lavender is Lavandula × allardii akin to the ‘African Pride’ type of cultivar, which has occasional leaves showing dentation in the upper half. Difficult to commit without seeing it in person.  The close-up of the leaves show the dense indumentum typical of these hybrids.  Paucity of flowering is also typical and since these lavenders are often sold as L. dentata in Europe, there are many disappointed customers about.  Often used for topiary.  Yes there is confusion over the heterophylla and allardii lavenders and I haven’t seen a helpful account yet.

Glad to hear your angustifolias responded to the cold, Marilyn. I hear that Olivier Filippi is recommending the chaytorae hybrids for places with warmer winters where angustifolia might not do as well; these are crosses between L. lanata and L. angustifolia and don’t disappoint. Like the allardii/heterophylla groups, they occur only in cultivation and as far as I am aware are sterile too. 

[MikeHardman]

Thank you very much Joan. I shall check the leaves tomorrow and report back...
Update: There is no dentation at all in any of the leaves.

[MikeHardman]

Well, my plants carried on expanding, and being wary of how big they might get during the course of the coming summer, we decided to cut them back hard. ...Which I know some lavenders do not take kindly to. We reduced them by 75% in height and width. Now, after a few weeks, they are sprouting strongly.

We will now give them another shock and move them to a wilder part of the garden, where we can see how they cope with competition from herbage. I know recent talk on this forum suggests they don't thrive in competition. We shall see in this case. In their new situation, their (L. x allardii) relative lack of flowers, larger habit and fly-attractivity will matter less. I have plenty of largish cuttings to put in, too, in the same area.

I will post a progress report in due course.