Claiming restrospectively

Hello, Another question! I was reading the posts on claiming late payment charges on already paid invoices. There seemed to be some disagreement if late payment charges could be levied retrospectively. In our case it almost always would be retrospective as if by the time we notice a late payer, give them a chase and then issue a late payment charge it is probably 30 days overdue by which time they might have just paid the invoice.
So I checked the Act and there is no mention of a time limit so referring to the 'User guide to late payment legislation' on the BIS website it said charges could be levied for up to 6 year retroactively. So my questions are:

1) If the ‘official’ BIS government website guide says its allowed, and in the absence of anything official to the contrary, why is there disagreement over the interpretation of the Act as should a government department not know their stuff?
2) As the Act has been around for a while is there not some case law where suppliers/administrators have charged retrospectively for 6 years, its gone to court and they have won (or lost)?
3) If retrospective claims up to 6 years have been disallowed by courts have they indicated what is acceptable i.e. is 30 days retrospective ok

Many thanks

The longer the delay in making a claim, the bigger the risk that the interest of your claim will be remitted.

However there is no mention in the act for the remission of the compensation due under the act.

Compensation is due on each and every invoice paid late and outside agreed terms.