Unknown find in the garden...

I found these egg cases in the garden on a Wisteria stem. I'd be really interested to know what type of insect created and hatched from them if anyone can tell me?

The whole structure is about 10mm long and 3/4mm deep. (I love the little 'lids' made by the larvae chewing their way out, whatever they were!)

Update (25/2)...

Through a contact I know from using Flickr, I have managed to get some additional information as to what this is... These are not actually egg cases as I'd thought, they are most likely the pupae of a parasitic wasp. I'm told that from the numbers present, the wasp larvae have probably emerged from some unfortunate creature and then pupated together, with the remains of the host insect having been washed or blown away over time.