I am American, but I think this is available in the UK.
We use cowboy magic, it’s is a product for horses. It’s a detangler and it truly is magic. A tiny amount really goes a long way too.
I have a long haired Rottweiler that gets super messy, loves to run around in tall weeds, get his chew treats all over his legs and neck, and generally just find messy things to get stuck in his hair.
I will brush him a few times a week and afterwards I’ll put like a nickel sized amount in my hand and run my hands through his problem areas, legs, neck, butt, and tail.
I’m able to the just brush most things off of him with just my hand, and things that would have gotten matted or stuck will just pull out. It also makes him shine really nicely, and I think it smells wonderful.
I would post on /r/findareddit, asking for a hair advice reddit.
Be as descriptive as you can of her hair texture - In the world of hair care, there can be pretty distinct differences in techniques and products depending on texture.
My only experience is with horses - I've successfully detangled manes and tails many times. The product I've used is Cowboy Magic Detangler & Shine. It's an oil type "finishing" product, smells nice, works really well, and a good quality product that overall helps hair. If you search Amazon for "hair oil", you'll see lots of similar products for people.
As for technique, the one thing that's really helped me is just leaving it in overnight. Of course, with horses, you don't have sheets and blankets and pillows and mattresses to consider! So maybe with your mil, apply in the morning, planning to do detangling in the evening. Anyway, main point is, when I would saturate, and then immediately go to work with a brush, I wouldn't get very far. But when I would saturate, then come back the next day with a brush, I could get tangles out pretty easily.
Anyway, hope that helps, good luck!
Edit -
I'm going to second /u/justmerc 's suggestion - My son had long hair when he was a kid, and I would do that very thing, just slather on the conditioner in the bath, comb it easily, then rinse thoroughly.
It's not a spray but I use Cowboy Magic on my rough border collie's coat and it's awesome. https://www.amazon.com/dp/B00M383SOC/ref=cm_sw_r_apan_glt_fabc_T6JG38EA1TR11GYF1SSV