Pat chicken dry inside and out with paper towel. Cut 1 lemon into wedges. Place lemon wedges and 4 thyme sprigs inside cavity of chicken; secure cavity with a fine skewer.
Make a pocket under skin of breasts, drumsticks and thighs with your fingers. Using disposable gloves, rub ¼ cup of the marinade under skin. Tuck wing tips under; tie legs together with kitchen string. Rub another ⅓ cup marinade over chicken.
Heat oil in a 5-litre (20-cup) slow cooker on 'sear' (HIGH) setting. Cook chicken, turning, until browned all over.
Adjust setting to LOW; cook chicken, covered, for 6 hours.
Cut chicken into pieces. Cut remaining lemon into wedges. Swirl remaining piri piri marinade through mayonnaise.
Serve chicken topped with remaining thyme sprigs, with mayonnaise mixture and lemon wedges to the side; season.