This is definitely an Acer, but what you're seeing there is a case of Rhytisma acerinum, commonly known as Tar Spot of Acer, or Acer Leaf Spot. It's a fungal infection, and in the UK, there is no effective fungicide treatment available, though you may find there is something you can use where you are.
This infection doesn't kill the tree, but it can make it look unsightly (as you've discovered). Some control can be effected by removing fallen leaves, particularly in Fall, because the spores over winter on the leaves. Removing the worst affected leaves might help, but I'm not convinced it makes any difference other than to reduce the aesthetic effect a bit.
UPDATE: Regarding sun burn from water spots, this isn't possible. I was shocked when I found out it was a myth, been warning people about it for years, but the original tests for this theory were performed with water droplet sized glass placed on leaves - the leaves burned. But when the same tests were performed with real water droplets, on both smooth and non smooth leaves, no damage could be reproduced. Water droplets have a different refractive index from glass, and don't maintain their shape, which isn't true of glass droplets - the water also cools the leaf surface generally. Further, I've tested it myself when the temperature on my very sheltered balcony has been 55 deg C in direct sun by placing droplets on a Foxglove leaf, which is ridged and not smooth. No damage resulted. Reference: www.AmericanScientist.org/issues/pub/sunburned-ferns.
UPDATE: Laughing-Jack may well be right when he mentions Phyllosticta - I jumped to Tar Spot, but actually, looking at it again, it might not be, but I'm 100% convinced this IS a fungal leaf spot - whether its phyllosticta or another type. I suggest you check to see if there are any fungal sprays or treatments you can use in your area - the UK is very short on effective anti fungal treatments, but maybe its better there. Point given, LaughingJack, though I'm not sure that counts for a comment - post as an answer!