Unlike EASA, you appear to be able to sit ONE ATPL paper (old Nav block?) at a time for CPL/IR holders.
And again like old UK CAA rules, no requirement to be a (f)ATPL for RHS >5700kgs.
I think that changed mid 1990s?
If I recall correctly, under UK CAA CAP54/509 [pre JAR/EASA] last century, you had three attempts to successfully complete the block.