Scummymummy, I think it's absolutely right to leave the room. I have been so fed up with DD occasionally that I have put her somewhere safe, cot or something and then left the room to compose myself for a few minutes before going in and trying to comfort her / start again.
There are a couple of things I want to say here. I know that my dd and I think most babies, react greatly to the moods / emotions around them. My dd senses when I'm wound up / frustrated and just gets more wound up and upset herself. She is only 7 months but has a real little temper on her.
This I know, when DD was 6 weeks old she started to scream everynight from 6pm and was INCONSOLABLE and I mean inconsolable. DH and I were almost in tears each night and would jig her around, pass her between us, try singing, playing, rocking, everythig for about 2 hours and the crying would just get worse. It was awful. We even called the doc out and he patronisingly told us not to be silly she just had colic. I swear if you were in our house you would have thought we'd chopped her legs off ! But you know what, it wasn't colic and I think this is a highly overused term. I tell you what it was. She was tired and she was trying to tell us she was tired and we'd missed the signs. She wanted to be put down to go to sleep and the more we tried to stop her crying by doing things the more overstimulated she became and the harder it was for her to drop off. Now she was desperately tired and just wanted to sleep.
Once we'd figured this out and had a routine she was so happy and content and now never cries except for a few minutes after her bath (hates getting out and being dressed ) and if she's overtired.
When she is too tired and won't give in / keeps fighting sleep during the day I will put her in her cot or outside and she will cry - I don't actually leave her but stay with her but won't pick her up, instead try to soothe her to sleep. It works 99% of the time. I won't get her up because I know that she is desperately tired and needs me to help teach her how to sleep. She will be so much happier when she wakes and she is always. We did this during the night to stop her wake ups. We'd go in, give her a dummy, stroke her head etc until she went to sleep. She would complain bitterly at first but after a few goes, didn't do it anymore and is so much happier when she's better rested.
I think another thing that should be mentioned that hasnt' been so far is that a mother should also know her babies cries and if something is wrong, should never be left to cry. I will always pick my dd up if her cry is anything other than her ratty, overtired, angry cry which she often does when she can't quite get to sleep for a few minutes before sleeping. If a mother knows her babies cries she knows if something else is wrong and if she needs to respond by picking up.