aits all in Lasors.
From personal experience of my exams running out, they have to be completed within 18 months of passing the 1st one. Then they are valid as a pass for the issue of a ppl for 2 years from the date of passing the last one.
My mistake was passing them all in 4 months and then due to work and the good old British weather, they ran out before i could get a test in.
The trick is to leave one untill just before your skills test or just before the end of the 18 month limit ( i would suggest HP & L ), then you still have 2 full years to complete.
By the way, i actually understood a lot more the 2nd time around so that was a plus.
Hope this helps, check it in Lasors at the CAA website, ppl section.