She's 3 1/2 months Pitbull, she's never had any problems with garden and never had a traumatic experience outside. I was thinking, maybe its because it's cold, but its only about 5 degrees centigrade. Ive tried using toys and treats but they don't work, she eats the treat, and then sits down. Ive tried carrying her round the block and trying to walk her, she walks, but leads me back home. Help much appreciated.

It very well could be due to the cold weather... It sounds like you're doing the right thing by "holding her hand" while outside.. If she doesn't potty after a good amount of time (however long you can stand being outside yourself), take her in and put her in her crate and try again in another 15 minutes.

Potty training is tough in winter.. You're going to have to just play it out and take it all with a grain of salt :) Pitties are smart dogs.. They learn kind of slow, but once they get it, they got it!

Good Luck.

She is training you to not make her go outside. You may have to start over, like when she was beginning training. Take her out on a leash to "the spot" and when she goes there make a huge deal out of it. Praise her like crazy then give her the treat. Be consistent.
Soon she will go back out and potty on her own but continue giving treats when she does.
I was a Vet Tech for 12 years and heard that question a lot.