I feel your pain joiedevivre... my 18 week old is exactly the same. I am totally despairing about her daytime naps. She sleeps for a maximum of half an hour at a time, it's almost impossible to get her to sleep at home, and she won't even go to sleep in the car. The only time she sleeps is in the pram, and if I stop moving she wakes up. Consequently I walk about 12 km per day. Which is fine when the weather is like it is today.. less good a few weeks ago when I was walking around the park in a blizzard.
The only way I get a nap at home is to swaddle, loud white noise, and rock and jig. This evening it took me 45 minutes of this to get her to sleep, and then she slept for 25 minutes. I've tried a mechanical swing chair, which everyone says is like a baby sleeping pill, but she doesn't sleep in it. She used to go to sleep in the sling (if I was walking around) but now she's bigger that doesn't work anymore. I think the problem is that she's super alert, and getting her to wind down for a nap is really hard. She just goes from playing excitedly to overtired with no wind down period. I've started to try to do a wind down, where I take her into the bedroom, draw the curtains, turn on some relaxing music or white noise and do some gentle baby massage, but it doesn't really work at all. I also spoke to my HV about it. She was helpful and suggested a 'gradual retreat' approach, where I put her in her cot and calm and soothe her until she falls asleep. This does work in the mornings, but not in the evenings. However, in the mornings it commonly takes her 40 minutes to fall asleep (with me holding her hands, stroking her head etc), and then she only sleeps for 30 minutes. It's really not worth it.
So yeah, I'm totally despairing and need help too! Dummies don't work - she spits them out. I think we're way beyond lullaby-singing sea-horses. I've also tried nap elongation techniques, but these don't work either. She's never ever nursed to sleep - or at least she always wakes up when I take her off the boob, and the last time she spontaneously fell asleep in my arms she was 4 weeks old.
One thing we have nailed though is bedtime. I suspect this was entirely accidental, but from when she was 2 weeks old, I started a bedtime routine where at 6.30 she has a bath, and then a feed in the bedroom with the lights dim. I found a 'sleepy baby' album on Itunes by a guy called Wayne Gratz, and we always always play that during bath time and the subsequent feed. At first I felt a bit stupid insisting on this routine because she'd be wide awake after her feed and not go to sleep until about 10 or 11 pm, but one day she just fell asleep on the boob at about 8. Since she's been about 8 weeks old she's gone to sleep no later than 8pm. At first she fell asleep in my arms and I held her until she was in a deep sleep then put her down, but now I put her down in her cot awake, shh and hold her hands, and she goes straight to sleep. Tonight she was awake when I put her down and it took about 1 minute for her to fall asleep. I think the routine really really helps.
Re your health visitor suggesting you let him cry.. most things I've read and most people I've talked to (and I've done a LOT of googling around this) suggest that there's really not much point letting a baby cry it out until they're 6 months old. It just doesn't really work that well. The longest my baby has cried for is 45 minutes (in the car seat, stuck in a traffic jam on the motorway...) and she worked herself into a terrible state and wasn't anywhere near to falling asleep. She did manage to scratch her head until it bled though. So no, I agree, crying it out is not on the agenda for me either right now.
Tips, suggestions and moral support greatly appreciated for me also!