Hi, your DD sounds like my DS2, who is now one. He is globally developmentally delayed, no known cause (but currently being investigated). At a similar age to your DD, he was similar, not holding head up, not really smiling, and very in attentive to toys.
He has Delayed Visual Maturation, and he was hearing impaired for a while (miraculously almost entirely "cured" with grommets, yes he had glue ear, and he was very young for the grommet op), and I really strongly believe that now that his vision and hearing have improved he is starting to come on. Still very behind, but he is getting there, with lots of extra stimulation (flashing lights in his eyes, etc), and physio.
We don't know what has caused DS2's problems...but he is coming along.
I hope you are able to get some good advice, support and information. When he was 10 months old (it took almost 3 months for the assessment to come through - what a waste of time!) he had a comprehensive assessment with a physio, OT, SALT, Social worker and pediatrician. Maybe your DD could have something similar.
We have found NHS mixed - great on the testing / assessment side, but almost non-existant on the therapy side.
All the best to you and your DD, it is such a worrying time, I still look back on his assessment in September as the low-point, but we got the report today (yes more than two months later!) and while it is depressing reading, it is already so out of date!
Good luck,
Oh, I don't know of any support groups for hypotonic babies, other than some on yahoogroups (easy to search on...)