As I understand it a SIM is basically just a foam pad stuck inside the outer covering. When you roll it up all the air is squeezed out of the foam pad. When you open the valve to inflate it again the foam naturally draws in air until it reaches it's normal shape. As far as I am aware there is nothing magical about it, so if you have damaged the outer covering any airtight patch will fix it.
A day of all things motorhome and campervan. The show is held both in the main indoor arena and across the grounds. Catering, hot drinks and food options are available. Easy access to the site and free parking included. Win one of five pairs of day tickets