So shall I let her sleep as much as she wants in day? Does sleep beget sleep?
She's only 15 weeks, so yes. Needing to limit daytime sleep is the stuff of the toddler years, not the baby years and absolutely not for the first 6 months. You are definitely over thinking here - your baby is not that far from being a newborn and it's to be expected that newborns basically sleep all the time.
So as soon as she showing tired cues pop her to sleep?
Yes. Put down to sleep at the first indication of a grumble, assuming you know the grumble isn't due to hunger or wind. It's a good habit to get into to feed upon baby waking up from a nap, then wind well. Then when you put baby down on the floor to play, you know that the next time baby grumbles it isn't due to hunger or wind, it must be tiredness.
With shorter cycles how do we help her go into the next cycle in the day - wud this be as you suggested tapping dummy close to her and such?
Yes. Key is to respond to baby before waking up. The sleep cycle will involve baby leaving the deep sleep phase and entering a light sleep phase. The aim of linking a sleep cycle is to go from that light sleep back into deep sleep. If baby wakes up from that light sleep, then the chances of going back to sleep are much harder.
So watch for the slight signs of moving out of the deep sleep phase. This might be something like an arm or leg moving, shuffling position, face screwing up. Start resettling then. If you wait until baby is crying or eyes open, then they've moved out of light sleep and are now awake - so resettle much harder.
Great tips with side cot, we have one that rocks - no rocking at all?
Moving is a great way to help baby sleep independently. So rocking the cot itself is fine. Just think forward with a view to planning to stop the need for movement (when moving into the big cot, for example). So rock to get baby off to sleep (and to resettle between cycles, if needed) but get her used to be stationary when asleep. Then consciously try to reduce the amount of time you rock and the tempo over time. So only ever give just enough rocking, stop as early as you can. Work toward just a quick back and forth rock needed to trigger sleep, then stationary. Then no movement at all. But it's a gradual change.
My OH says I shud let her go bed at 10pm to go through till 7am ish this isnt right?
Your baby should be in the same room as you for all sleeps. If you and DH are both downstairs until 10pm, so should baby. This is in line with the Safe Sleep guidance here:
www.lullabytrust.org.uk/safer-sleep-advice/room-sharing/
It is therefore normal, and what most families do, is to continue baby "daytime napping" right through until you go to bed, say 10-11pm. This doesn't mean baby is awake in the evening, but that the sleeping is happening downstairs with you, as with daytime naps. Then at the wake up that happens at around 10-11pm, give a feed if needed (to prolong the next stretch of sleep) and take upstairs to settle "in bed" as you go to bed too.
Can I work on night time sleeps first and then the naps once she mastered the night times?
There's no massive distinction. I think you're thinking far too deeply here. Develop good sleep habits for both nights and daytime naps. Always aim for more sleep, not less.