I'm retired, do I need to enrol on a Professional Competence Scheme?

Yes 

As long as you hold Medical Council registration on the Supervised, General or Specialist Register, you must be enrolled on Professional Competence Scheme.  Your legal requirement to engage in Professional Competence is tied to your Medical Council Registration, not your employment status

Related Articles:

What constitutes Engaged in the Practice of Medicine?

I am engaged in the practice of medicine, what must I do every year?

I am not engaged in the practice of medicine, what must I do every year?

See: Medical Council information for retired doctors who continue the practice of medicine on an ad-hoc basis.