Thanks to you all. We have made some big changes and things have been a lot better the last 2 nights so fingers crossed this will continue. In case this is helpful to anyone else, here's what we did...
Removed swaddle and paci:
Both DH and I both thought that some of the reason for DS2's NWs was that now that he is getting more mobile and more in control of him arms that he objected to being restrained. With this in mind, on Tuesday during the day, in a desperate attempt to make some changes, I tried (again!) to remove the swaddle. When I did this, I found that he kept removing the paci with his hands and the flinging the paci and his little arms about. He was never a real paci baby and he would always spit it once he fell asleep so I figured that it was probably acting as more of a physical distraction rather than having any pacifying effect. So I decided that if we were going to remove the swaddle, we would have to remove the paci too. Too my surprise, he settled relatively easily (15mins pat/shush) for his second daytime nap and his catnap.
Extended A times:
On JAY3's suggestion, I extended his A times from 2 hours to 2h15-2h30.
Reduced amount of solid food:
On Shiv52's suggestion, yesterday I reduced the amount of solids I was giving him in favour of concentraing on getting closer to 900mls into him during the day.
Persevered with pat/shush:
I had been reluctant to use pat/shush for that long at nighttime as I was worried that DS2 would wake DS1, but the past 2 nights I have persevered with it while knowing that he was getting his ca. 900ml and didn't NEED the extra calories in a nighttime feed. Surprisingly DS1 hasn't stirred at all during my nighttime pat/shushing!
Results:
Last night was the best we have had in quite a while... we just had one waking 5.15-5.25 and just the 10 minutes of pat/shush sent him back to sleep until 7.15. In general, DS2 seems much happier and is napping lots better in the day also, now that he has "relearned" to self soothe without the paci and swaddle.. This afternoon he put himself to sleep without the paci, the swaddle or any intervention from me. I do feel that we have made some pretty big changes but the results have been great.
Thanks again to you both for your suggestions. We seem to have solved our problems for the most part (at least for now, anyway!).