I have may of missed it in the rules but if I roll multiple advantages can I use them for multiple crits in combat i.e I hit with a blaster rifle and score a success supported with six advantages - can I pay for two critical hits ?

No you add +10 to the crit roll for each additional crit you inflict with one hit.

Many thanks - found the rule now on page 158 - cheers.

However, if you hit them multiple times in one turn, like if you are dual wielding or using the Auto-Fire quality, you can trigger 1 critical injury per successful hit!