busywizzy, sorry I haven't replied for ages. I haven't been on mumsnet for a while, too much going on at the mo. Don't know whether you're still having problems.
Well I thought I had it cracked! Now we're back to square one. She's teething and I think she might be having a growing spurt as well. Could your ds be having a growth spurt? Is it hunger keeping him awake? Or could it be tummy ache, if you've just started weaning him? It could just be he's got into the habit, like you say, I found this happened after my dd had a cold. But you need to eliminate any other causes before doing PUPD.
What I'm discovering also is you think you've got them into a routine and then it all changes- I guess that the nature of children- always growing and changing!
PUPD is a useful tool, once you've eliminated genuine hunger, pain, feeling too cold or hot or anything specific that might be waking them. The babywhisperer says that generally if they wake at the same times every night it's probably habit, not hunger.
The PUPD varies slightly depending on age. For a 6 mth old, if he's burrowing his head in the mattress, turing his head from side to side, getting up on his knees or flopping from side to side, don't pick him up right away. Talk reassuringly. Reach out to him and see if he responds. When you pick him up, only hold him for a couple of minutes then lay him down, even if he is still crying, otherwise he will learn to cry to get picked up and held. Then pick him up again and repeat. If he flails around hold him firmly but gently in a cradled position to help him calm down.If he fights you lay him down, but pick him up again straightaway. You are giving the message, I am here for you but you can learn to go to sleep on your own. Give quiet verbal reassurance throughout. See if he can settle himself, if not carry on. The first few times you do it, it can take ages, and it's exhausting, but you need to be consistent and see it through each time you do it.
If you want any more info, get the book it's brilliant, she goes into a lot of detail for different ages of babies and toddlers, about sleep, routines, eating, behaviour. It's called "The babywhisperer solves all your problems" by Tracy Hogg with Melinda Blau. I ordered it from Amazon.