Switching to smarty and keeping my number when i have already been given a new number by smarty

Have you given the PAC, from your old provider, to SMARTY yet SimonSc_1692145?

When you get a new SIM from SMARTY it comes with a new mobile number. To transfer and keep your mobile number you need to give SMARTY a PAC. 

The procedure is explained online at https://help.smarty.co.uk/en/articles/1062823-transferring-my-number.