We got him from the RSPCA. He's a Jack Russel Terrier cross. They know nothing about him though they reckon he's 1.5 to 2 years old.
He's very bright and very amenable indoors. He got 'sit' and 'stay' on day 2. However, when we let him off the lead in the park he just goes.
We take treats. We have a whistle. We've tried doing recall training whilst in a fenced off tennis court. He will come back then.
Today I chased him through rose bushes and all over the park. He was often within grabbing distance but he just wouldn't come. I stayed as patient as I could and called him in a bright and cheerful way but he just looked at me as if to say, 'you're having a larf aren't you?'
I can't have a dog who stays on the lead all his life. I am out of my depth here. If I can't get him to come back he will have to go back. 
Any tips? Success stories? Throw me a bone.