My 5month old baby does not sleep well at night atall. We've tried all sorts, feeding her right before she goes to bed, music, changing her, a sleeping bag to make sure she's not getting cold, a new matress for the cot but nothing seems to ease her.
The annoying thing is that in the day she sleeps fine, even through her 3year old brothers shouting and yelling but during the night she constantly fidgets and tosses and turns like she's never fully comfortable.
If anybody could give me any suggestions I'd be so grateful. Thank you

How long are the stretches of sleep she has during the day? If more than two hours, you may want to try to wake her after that.
Is she just fidgety at night or does she actually wake frequently? Do you go right in when she does wake? Does she still have a feed or two during the night?

Has she been learning any new skills recently? Sitting up, rolling around, getting more mobile? Cutting any teeth?

Sometimes babies will wake more for a bit when they are going through another stage of development.

A few things you could try:

*kerb her sleep a bit during the day if she sleeps long stretches or frequently
*have a bedtime routine if you don't already... a bit before bedtime, bath her, feed her, wind her, a cuddle, and down to bed
*keep any nighttime interaction to a minimum... not much talking, low light, quick change, feed, cuddle, back to bed
*make sure she is fully awake before going in at night... just as we do, babies will wake and move about during the night and then settle back down... they often will cry out a bit as well. If you know she is not likely hungry, see if she will settle on her own before going in.
*if you think she is uncomfortable, maybe have her last feed a little before bedtime. Sometimes a feed right before bed can cause discomfort when lying straight down.

Thank you so much for your advice DoulaLC. I will definitely try your suggestions.