I am new here & am hoping someone can identify with my worry. I have an adorable Old English Sheep dog - 2 years old and the light of my life. She has had 3 seasons [just finishingf the 3rd] and she seems to be incontinent of urine when in season. The 1st was pretty wet, the 2nd not too bad but this time [3rd]as bad as the 1st. She seems unaware and untroubled. It happens at night as her bladder fills & then she dribbles - when she is out and about and weeing as necessary all seems fine. My vet does not seem to have an answer - she is fit, healthy & happy.

Has any one else had this experience - has anyone any suggestions?

Like humans, dogs can get gestational diabetes, and believe it or not, the sugar levels can change in a dog, during the heat cycle. It could be a short term diabetes thing that goes on, and the only way to stop that is of course spaying. If you are wanting another heat, I would have her checked THEN, for diabetes, and that would then be ruled out.
I know alot about diabetes as I have had it myself for 34 years, and have always listened to human and dog stuff about it. Hope that is not what the problem is, because frankly, it is not a good thing.