I recently received a rather odd letter from the WDA about the Skills Development Levy, addressed to my company complete with a case number. It states that all employers have to pay SDL, provides details of how it's calculated and paid, says that employers may be required to furnish information on their SDL liability by returning the attached form... and then finishes off with "please ignore this letter if you have confirmed that your SDL is correctly computed and paid". Note total lack of detail on what liability they think I may have, much less why or how much it is.

Now I'm already paying the SDL for all my current employees as part of their CPF contributions, and have always done so for all SG/PRs. However, in the past there may have been EP employees who did not contribute to CPF, so their SDL paperwork might have fallen through the gap.
Has anybody else gotten one of these, and what did you/should I do? Am I supposed to ask them to confirm that my SDL is correctly computed and paid (how, and will they? Knowing Sg bureaucrats, they won't!), proactively admit to a past failing and send my best guess at the owed amount, or just ignore the letter and see what happens?