Thanks for doing this thread, it's great
You provide ample caveats above, but on this point:
The issue with a retrospective change is, I believe, that the fee payer has now made an assessment, as is their (new) responsibility, whereas the old assessment (and corresponding action) was the responsibility of the PSC (and by extension, its director/s). I assume that's what you mean by "retrospective", i.e. an engagement that straddles the old and new legislation. If you mean a retrospective change post April 2016 (i.e. post the new 10(2) of the ITEPA), then I agree. At the very least, I think it's worth making this distinction. Post April-2016, it's the responsibility of the fee payer, end of story, and any attempt to claw this back, even with a contractual clause that might allow this in principle, is, in my opinion, likely to fail.My feeling would be that the past tax must fall to the client / fee payer because they did not take reasonable care and the error was not made in good faith, so the dividends already paid would be allowed, the fee payer should pay the tax, but from this point on future invoices will be paid net of income tax and NI.
If you're referring to a change in assessment that straddles the change in legislation, I would view the situation as being for the PSC to correct any historical error, assuming it needs to be corrected at all (because this could be disputed). Further, I would view any attempt by the client to operate a deemed payment, retrospectively, when it was the responsibility of the PSC to do so would be not in keeping with the ITEPA and hence unlawful. But if the old assessment by the PSC was, in fact, incorrect, the responsibility would rest with the PSC to correct the old accounts and pay any tax/penalties due for their incorrect assessment. Again, my opinion is not worth any more than yours, since I'm not an expert, but the above seems more logical to me.