Of my litter of 7 - just one does not seem to be doing so well.
It has been suckling but has not put on weight and has lost since yesterday.
It is moving around but often seems to be on its own. Sometimes it has buried itself under the blanket on the heatmat. The others do not do that. They just pile up and seem to be normal.
I don't want to over describe the situation but when it walks "sometimes" it seems to crawl with its head on its side rather than forward.
Has anyone ever experienced a puppy like this. Not sure what to do,

I don't want to alarm you, but I would be alarmed.  Very alarmed.  If it were my puppy, I would take it to the vet immediately.  The puppy may have to be put down.  It is a little early for neo-natal ataxia, I should think, but "flat puppy syndrome" might also be involved.  You need medical advice.  I do hope things turn out OK.

Have you got puppystim?   I would give some little before feeds to give him/her a boost and make sure the puppy is near the back teats so will get nice amount of milk easily... not sure of the other stuff going on... but doubt vet would know to do much with puppy so young, espically if nothing is very obviously wrong e.g puppy in discomfort or clearly just going downhill or constantly moving wonky. Could just be the puppy laid in award position in the womb and will straighten out in few days...

I would try fluids under the skin.  It could not make it worse.  Once they go down hill and get dehydrated it is very dangerous.  I would not give it too much hope.

OK I have been concentrating on this puppy. Making sure it has the best position and is not knocked off.
Also making sure she is not cast out of the main heat area. I did wonder if she had got a bit chilled and being that much smaller was not able to deal.
She seemed much stronger over night when feeding and has put on 10 grams but 35 grams from the point where she lost. She also seems to be getting about much more normally.W
Will see how to-day goes first.

Puppies often do lose a little weight after birth and in itself that would not concern me, I weigh at birth and not again as I think you can tell if progress is satisfactory by behaviour, quiet and content and also looking plump and full.  Keep an eye on this little one and if she feels cooler to the touch, gets a noisy chest or is making strange noises then pop her to the vet's, looking at her in comparison to the others is a good guide as to whether there is something wrong. Good luck and keep posting.