Looking at your exams, as you have not completed them in an 18 month period, you will have to retake Air Law as you say and Navigation also runs out on 31st December so you need to retake air law and pass the Flight Planning by that date.
As you have passed both the RTF Exams you can apply for the FRTOL free of charge at any time
See FCL508 SRG1106 Section 5 and Note 4 or you can wait apply for the FRTOL at the same time as you apply for the PPL when complete. Either way the FRTOL is free.
Pre JAA the exams were only valid for 12 months so students were always retaking exams. The current system is unbelievably relaxed.