Hi Snowbunny,
Our son has suffered with flat head syndrome. I would hate for your newborn to have to go through the turmoil we've been through all because we listened to our GP who said the same as your HV. Which by the way is the standard response from most HVs and GPs on this matter.
In most cases it does not just go away on it's own it gets worse. Your baby is at the perfect age to prevent it from getting worse, what others have adviced on repositioning is really important for you to do.
When I looked back at our photos I realised that our son always slept on his left hand side and sure enough that's where he developed a flat spot. The GP said at his 8 week check that it was nothing to worry about that it would grow out. So I took her word for it and didn't take any action until we realised a few weeks later that it had worsened.
A search on the internet led me to 2 brilliant plagio forums and we found excellent advice on repo.
http://groups.msn.com/PlagioUK/plagiouk.msnw
http://www.plagiocephalycare.org/board/
We started repo from 14 weeks (9 weeks corrected age). We bought goi goi pillows for nappy changes, a sleepcurve for his cot when he'd moved into his cot at 4.5 months. It helped stabilise his flat head but it's best to use them from the off in a moses basket to help prevent plagio from happening in the first place.
Our son didn't like tummy time when he was very young either but we kept trying him. There are some toys around to encourage tummy time, like the ones with water in them or any toys with mirrors on that kind of thing.
Try using a bumbo seat when he's a litle bit older and a door bouncer, also a sling. It's not easy at all trying to find ways to keep them off their backs, it's hard work but worth it to stop the flat spot from worsening.
I've seen so many babies at various baby groups with really bad flat heads, it's such a shame the NHS don't give out more advice about the importance of tummy time and repositioning. Not just tummy time.
Our sons head did improve with repo when you looked at his head from a top down view but unfortunately it caused more of a flatness across the back of his head (brachycephaly). In the end we've had to resort to helmet treatment as we couldn't bare the thought of him being bullied for having such a flat head when he's older.
He's due to complete treatment in a weeks time after only 7 weeks and his head looks fantastic now.
I wish I'd had more awareness of it like you but at last the nightmare is nearly over for us and we have a baby with a normal looking head now