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The timing and some of the details are a little off, but could it be Moondial? https://en.wikipedia.org/wiki/Moondial_(TV_serial)
Here is the list of all CBBC shows: https://en.wikipedia.org/wiki/List_of_BBC_children%27s_television_programmes#Live-action
And I don't think it's the one called Harriet's Army.
OH MY GOD; MY PARENTS ARE GARDENIIIIINNNNGGGGG!!!!!You're sure it was a children's show, then? It wasn't Marchlands, maybe...? https://en.wikipedia.org/wiki/The_Oaks_Trilogy#Marchlands
Maybe an episode of Shadows? https://en.wikipedia.org/wiki/Shadows_(TV_series)
It's much older than OP's stated timeframe, but y'all might be thinking of something different, or there may have been reruns at the time OP remembers. OP's description also sounds ssssort of like the novel "When Helen Comes," but that doesn't have any filmed adaptation that I know of, so unless they're blurring details across that book and a show...??
I remember a BBC schools mini series called Shadow Play: A Victorian Mystery about a boy called Ben who used a camcorder to see into the Victorian times when he was in an old house. He sees a ghostly woman in some of the rooms (which really creeped the heck out of me!) and later on he was able to travel back in time and meet a girl named Hester whom he helped. Like you said, her mother (the ghostly woman) drowned in a lake. They released a book to go with it, which I actually have.
https://www.google.co.uk/search?q=shadow+play+a+victorian+mystery&client=tablet-android-samsung&prmd=visn&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjp8aekmtnTAhXIC8AKHedbCJUQ_AUICigC&biw=601&bih=962#imgrc=3geBPqpOSna-AM:

This is been bugging me for years but I remember this show that was on cbbc on the 00s, I'm not sure it was a mini series or a tv film. It was about a girl and her family who'd moved to a house near a lake and in the house she'd found a necklace which allowed her to talk to a Victorian girl who used to live in the house. I don't remember too much of the plot expect a big part of it was about the Victorian girls mother who died when she drowned crossing the lake after no one lit a candle for her to see her way across. The modern girl helped her fight the answers about her mother so she'd be at peace? I'm sorry that's all I remember :(