my own SIM is an old one with the large gold bit and it works fine trimmed down.
I just used a free multi SIM from another network. Popped out the micro size then held my SIM and the other (larger bit) together. Drew around the gap (where the micro SIM was) with a pen then cut it out with a pair of scissors.
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Just an update, after a lot of research I finally plucked up the courage to cut it. As I was loading my phone up after I had done it I thought that it would be just my luck that it didn't work but to my surprise it did! I'm so chuffed. Thank you for your help!