An AIC on this subject was issued last April. Since then ALL applications for exams including resits have required the signature of a JAR approved groundschool.
To start a second sequence of exams you must undertake a minimum of 10% (60 hours) of retraining. This does not need to cover all subjects, but must be agreed and provided by the school signing your application forms.
You can start the retraining and second sequence of exams as soon as you like.
If you do start again it is probably a good idea to do your problem subjects first. If you cannot pass these there is little point in redoing the "easy" ones.
You should also think long and hard before trying to do the minimum 60 hours retraining then doing the rest without the support of a school. I am not saying this simply because I work for a school, but because I have seen too many students who have wasted most of their 18 months working alone at home, before coming to us for their last gasp attempts.
If you decide to start again make absolutely certain that you intend to do whatever is necessary to succeed.