It CERTAINLY sounds like silent reflux - I wouldn't change their milk personally I would go back to the GP and either see the same on or a different one but DON'T let them send you away without a prescription! My HV prescribed it for max at 10 WEEKS (as you know he was prem too so he was only 4 weeks corrected - same as your boys) so there is NO reason why you can't have it to add to their milk.
The refusing milk, the gurgly tummy, the arching back, the screaming are all PERFECT symptoms - REALLY TRUELY DO NOT BE FOBBED OFF OVER THIS, WE WERE FOR AGES COS MAX WAS STILL PUTTING ON WEIGHT ETC ETC BUT HE WAS LIKE A DIFFERENT BABY ONCE WE HAD IT!!!!
Hope you get sorted!