UPDATE: things have moved on since my last update. Many professionals have been talking to HMRC at quite a high level and two of my newer cases have been allowed (I have been advising another 6 or 7, at various stages of the reconsideration/appeal process - and I lodged two Tribunal appeals last week)
As regards my original case, it was at the Tribunal yesterday and I believe it went quite well. There were two main arguments, as well as the general unfairness of HMRC changing their policy. (They deny that, of course.)
1) The legislation states (loosely) that the claimant shall furnish "a Certificate of Completion, or other evidence such as HMRC might require. The judge raised the point in a slightly interesting way, that you might be able to rely on the Cert Of Completion (if you have one) and not need to bother with any other form of evidence. That fits in with HMRC guidance - 5 mentions of Comp Certificate, but only one mention of the other forms of evidence.
2) The amount of work that was carried out after HMRC's notional completion date. There was quite a bit, in this case.
If we win on point 1, it is likely to assist other claimants; if we win on point 2, it won't, as the general rule with Tribunals is that they are personal to the claimants and their individual circumstances. Each claimant will have slightly different, or vastly different circumstances, so it's certainly worth considering the Tribunal route
And if we lose, I reckon that the next step will be complaining to HMRC, as one of the biggest injustices here is that everyone affected has adhered to the to HMRC's guidance in the VAT 431C and 431NB. As well as going down the reconsideration/Tribunal route, I've also been advising clients to complain to HMRC (and their MP) in the first instance and if that fails, complain to the Adjudicator's Office. Being misled/misdirected by HMRC isn't really a Tribunal matter - they can only deal with the law.
I'll keep you posted.