It really comes down to what you and your child are comfortable with. Start with small increments of time and gradually let him stay alone for longer and longer periods. If he's mature enough to handle it you'll know. Make sure you set reasonable rules (no cooking, no one comes over, etc.) and that you're accessible by phone if he gets nervous or has any questions. Until you're both comfortable with it, don't travel too far. Not sure how long it takes you to nip in to Sainsburys, but if you can be home in 10-15 minutes in case of emergency, or whatever time frame makes you comfortable, start there.