In the past I have been very susceptible to recurrent mastitis, does this have any relevance?

Mastitis means bacterial infection of the breasts (not the same as mastalgia which is pain in the breasts or inflammation sometimes associated with the menstrual cycle). If it is mastitis, this may be important. If this is spontaneous and unrelated to pregnancy it is quite unusual We would need to have a conversation about when and how often this situation occurs prior to deciding upon breast implants.

Putting the implant under the muscle would lessen the chances of infection of the implant during mastitis but if this is a frequent problem I would not recommend any form of breast implant and you will need to be investigated as to the cause (unless this is related to pregnancy or breast feeding).