I used to train with Graham years ago at Dobbin Hill/Greystones now train with Farhad at Sheffield Kickboxing Club which is on Sidney St, I can highly recommend it.
I have also trained at Lotar's Club a few times which is another great club, down to earth and really good tuition.
I have also done one of the interclubs at Thanapron not trained there but they seem like a sound bunch and the tuition is supposed to be spot on. There are a lot of good clubs in sheffield so try a few and see which one suits you best.
Rob