give customer services a call on 789 from your mobile (0345 6000 789 from any other phone) & request a replacement SIM.
Tell them that you need to change your SIM size to "micro" or ask for a "multi SIM" then you can just pop it out to the size.
Once activated it will replace your present SIM, retaining your same mobile number and tariff / ballance.
However, you will be responsible for transferring any SIM content such as saved contacts. (There are various ways of doing this depending on the model of your phone's or preference).
Alternatively if your present SIM is the standard "mini" size, you could trim it down to "micro" size yourself (sounds scary but really easy). This is what I did and of course all SIM content is still there then
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