OOH GP/ECP work can be in walk in centres, cars in the community, telephone assessments or OOH clinics (by appointment).
With the good companies there will be a good support network if you are unsure about diagnosis on a particular patient and organisation will support you in second opinions but don't expect this to be over used otherwise they would question competency in the role.
ECPs do have weak areas that are usually addressed by the company with additional education and support palliative care, paediatric ailments (non-acute) and things like rashes.
Expectation of earnings range between £50,000 and £104,000 per annum
SERCO - http://www.serco.com/careers
Hallam Medical - http://www.hallammedical.com