UK company directors now living in France, trying to understand how to fill in the tax declaration

OK, so here is our situation…

We (myself, my wife, 2 kids) moved to France last year (end of August). We have opened up a SARL, which was registered in November. Before, we had a UK-based company, from which we paid ourselves salaries, and took dividends.

We’re trying to understand how to fill out our tax declarations properly. I know we are late, but I only got my social security number a few days ago (the day of the deadline). We have a UK accountant who has done our UK accounts, and a French accountant for our SARL, but who doesn’t seem willing/able to advise on the personal tax declaration issues as well.

We are getting conflicting info from the French authorities and our UK accountant. France is saying that our dividends should be taxed in both countries, but we would get a ‘credit d’impot’ to help soften the blow, and that we should claim tax back from the UK. Our UK accountant says that we should be paying the taxes in the UK and that the double taxation treaty means that we should declare it in France but that it shouldn’t be taxed here again.

So, the questions we have:

1 - Is it possible to get reimbursed from HMRC on the basis of the double taxation treaty?
2 - Is there a trusted/recommended accountant who could help us work through these issues so we get this right? Ideally, someone who has Belgian experience as well, as, to make things more complicated, my wife is Belgian and there are similar issues there!
3 - Does the fact that I didn’t have a social security number until the day of the deadline mean that we might be able to argue against potential penalties?!

Thanks!