Appointing a Subcontractor without prior acceptance is not a Defect as it's not "a part of the works not in accordance with the Works Information" as required by clause 11.2(5).
If the base course is not the required thickness then it is a Defect and should be notified as such by the PM under clause 42.2, even though you are close to the defects date, and it should be corrected by the C under clause 43.2 within the defect correction period.
There is a procedure for accepting the Defect at clause 44 which could give the Employer some compensation for the Defect in terms of reduced Prices and / or an earlier Completion Date. Either the PM or the C can propose a change to the Works Information to accept the Defect so if the C hasn't proposed it and the Employer is happy to consider it then the P should do it.
One thing I'm not clear about is your use of the term "sector scheme approved" which I don't understand, however I don't think it has much bearing on the issue. Focus on either correcting the Defect or accepting it, if you have concerns about the Subcontractor being proposed for correcting it then by all means discuss this via an early warning and risk reduction meeting and if necessary increase the level of supervision to ensure their remedial works aren't defective. If you refuse to allow the original Subcontractor to return to correct their Defect you may be conceding a compensation event for the additional cost to the C of having it corrected by others.
If the base course is not the required thickness then it is a Defect and should be notified as such by the PM under clause 42.2, even though you are close to the defects date, and it should be corrected by the C under clause 43.2 within the defect correction period.
There is a procedure for accepting the Defect at clause 44 which could give the Employer some compensation for the Defect in terms of reduced Prices and / or an earlier Completion Date. Either the PM or the C can propose a change to the Works Information to accept the Defect so if the C hasn't proposed it and the Employer is happy to consider it then the P should do it.
One thing I'm not clear about is your use of the term "sector scheme approved" which I don't understand, however I don't think it has much bearing on the issue. Focus on either correcting the Defect or accepting it, if you have concerns about the Subcontractor being proposed for correcting it then by all means discuss this via an early warning and risk reduction meeting and if necessary increase the level of supervision to ensure their remedial works aren't defective. If you refuse to allow the original Subcontractor to return to correct their Defect you may be conceding a compensation event for the additional cost to the C of having it corrected by others.