Ideal weight? Can this be varied? Help required.

Hello!  I wasn't sure where to post this! 

I just have been thinking whether I am at a healthy weight or not.  Firstly I exercise a fair amount... I'd say on average 10-20 hours per week (though I'll have some weeks off and I'd say perhaps a quarter to half of this time is fairly intense) but don't really regulate my eating habits... but I eat fairly healthily I think in regards to meals: lots of salads.  I don't eat meat, but do eat a lot of fish.  I don't think about how much of anything I eat at all though and I really don't want to get myself into counting calories or anything like that.  And being quite busy, I often don't have the time to think about such either.  I do also like my chocolate and ice cream and eat lots of that too!  I enjoy working out and I enjoy food! 

Anyway, I am just over 5 foot tall and weigh around 130lbs which does, to me seem to be overweight.  I know I weigh a lot more than other people I know of my height.  I would seldom mention my weight to anyone, but if ever I do, people seem to not believe I weigh so much (perhaps it is as it's likely mainly in my lower half?) 

I think I am also of small build going by wrist measurement scale (my wrists are around 5.5 inches and easily overlappable  spanning them) and how I was when younger.  However, my measurements are 32-27-42 (measured at the widest part).  My weight can fluctuate, but this seems a weight I keep coming back to and given that my thighs measure 26 inches - just an inch smaller than my waist I have found in the past that if I am under 105lbs, which I have been, it's really too little for me.  I'm 31 now (scary!) but back as a young teen did lose a little too much weight (I was under 6 stones for a short time, which is hard for me to imagine now!) and I don't want to get myself back to that, but I do recall I felt good at around 115lbs and am wondering whether I ought to try to return there and if so, if anyone has any advice as to how to get there and more importantly maintain that kind of weight?  I don't mind having big hips as I figure it makes your waist look smaller... but I would like to be healthy!

In fact I don't want to get into regularly weighing myself really!  I suppose I've come on here as it was maybe a year or so ago last time I weighed myself and then I was about 120lbs so I've put weight on evidently in the past year, yet I'm not sure why.

I don't know whether it's what I'm eating (probably!) or how I'm working out (I'd say I get most of my intense exercise in classes and using the step-climber-y thing in the gym... I really find it hard to run on track or treadmill at any speed or for any length of time but I can keep going at any other excercise!)

Obviously, given my sudden thinking this, I've looked online, and even according to this site, as I register, my ideal weight is 104lbs.  In which case, I would certainly need advice as to how to get there!

Thanks in advance for any advice :).