struggling with dogs aggressive behaviour

[tinypie]

hi, joined few mins ago. im hoping theres sumone out there with some much needed advice and tips on my dogs aggressive behaviour to other dogs. i have a 4yr old bitch englishbullmastiff called sally, she has very good manners, hardly ever barks and is brilliant on the lead she has an excellent tempremant with adults and kids of all ages, her only problem is she is extremely aggresive to other dogs, as soon as she spots a dog she attempts to make chase and go in for the kill, she,s only been like this 2 yrs after she was attacked and pinned down by this other dog, im constantly keeping an eye out for dogs in the distance so i can change direction from then to continue our walk as once she gets a few feet from the other dog she tries to pounce and grab them by the kneck, ive resorted to only walking her after 11 pm when i know its a slim chance of seeing another dog, she;s never of the lead to cause any damage, but has escaped out the garden franticaly because she has heard a dog approaching, the two incidents were very upsetting to me and the other owner and resukted in myself paying over £350 in there vet bills, i just want her bak to her normal playfull self and just to walk on the opposite side to a dog wud be great, HELP lol.