my retainer wont fit anymore?

i live in london so dental care is free basically. i used to have braces and was given on eof those clear retainers. i found that when i didnt wear it two of the teeth would go back to the normal state. anyway, now the retainer wont fit because i have grown more teeth at the back which irritate me when the retainers put on. what should i do? im going to call them up tomorow morning but does anyone know what they will be able to do to help?!
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They will probably need to give you a new retainer... and if the new teeth are wisdom teeth, you may need to have them pulled
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