I didn't see what happened as it was only just getting light, but I could make out that she had put up a hare (none usually live here, it must have moved in recently) and then she stopped dead and I could hear her howling in the dstance. I wasn't too worried at this point, because she is the biggest wimp ever, but as I got close to her I could see she was on all three and her front left leg was flopping as she limped 
.
So having got her home (thank goodness it was just around the back of our house) we called the vet and my husband took her to the clinic. Vet says that the break is just above her wrist/knee and should be fairly straightforward to fix.
She loves to run, it makes her day to have a proper, top speed gallop, but her legs are so fragile.
And I'd only just shelled out £165 to get New Dog neutered yesterday so my bank account is going to take a proper hammering this month.