Welcome Wez.
Ok sounds like you may be overfeeding Jazz., this will cause the runs.
She will eat as much as you give her, not her fault she's a puppy and hasn't learned to stop when she's full.
Yes change from the Bakers it's well. . . . not very good.
Most Tescos stock Wainrights or Harringtons, that's about as good as you'll get without going to a proper pets store
Pets at Home or Jolleys.
There have been a ton of threads about what is the best food, have a search.
For now though.
Starve her for 24hrs (yes she'll scream and whinge and cry, coz she's hungry and that also means no treats)
make sure she has plenty of water.
Then. . . Boil up about 50g of rice (let it cool)
and give her this with a small cooked or uncooked chicken breast.
do this 3 times per day until she gets firm.
Introduce your new kibble (a small amount at first) let her system get used to the new food.