As proposed the number was within area as well, so should have been plenty. However things have moved on, and I think you have intuitively assumed what is now the latest thinking. That is to remove the area code from the SID, so having a unique number within the county code. (If people wanted the mnemonic value of an area it could be optionally appended, but wouldn't contribute to the uniqueness of the SID)Footleg wrote: 1) Limiting SIDs to four digits assumes that there will be less than 10000 sites per county. I would suggest it safer to use five digits for future proofing the system.
The problem with the areas is that they would need to be defined, not easy. They also tend to change with time and fashion, also bad. At least the county boundaries are established and they don't change too rapidly.
Also counties like Devon don't really have area's with a sensible number of caves in them.
As to the number of digits, yes it needs to be longer. Lets say 5. That's 100 thousand sites within a county.