LondonMummy - I'm sorry to hear that you're feeling like you don't quite fit in with the NCT crowd.
I found out when my DS was little that just because you are mums, doesn't mean that you have to be friends. That total cow from school that just had a baby - it doesn't mean that you suddenly have loads in common, and yeah, she's still a total cow.
With regard to their super tidy houses. They most probably have cleaners and spend the week leading up to your visit making sure that you go away thinking they're Super Woman.
Unfortunately, our children often mirror our moods - so if you're stressed or anxious, they're going to mirror this and be mardy and need more attention. This means that when you're in a good mood and calm and chilled, your LO is likely to also seem content. This also means that in my house, PMT week is a battle from start to finish!
Try not to worry about it - leave them on good terms or suggest that you're doing such-an-such if anyone would like to do it with you and go your own way. Perhaps when the children are older and have more individual interests such as sport and dancing, you may meet up with them again and it will be nice to see some friendly faces and a friendship may have a better chance of blossoming.
There are plenty other groups and activities, and websites such as this and Netmums, Cow&Gate and FB groups are a good way to meet other local parents.
I have always worked better with 1-to-1 friendships but now that my DS is 6 and there have been that many birthday parties and the children have all started school, it's quite easy because everyone knows everyone and as I have lots of little friendship groups (a group of 2!) - there's no bitching and falling out :)