The dreaded Impots

I know it's a bit late to be asking about this, but I'm totally confused by all the gobbledygook I'm reading about double taxation not being allowed, although it seems the French tax authorities are allowed to tax it anyway?

I've been lucky in getting Tax Payable in France status on my pensions, but my husband has two government pensions on which he pays tax in UK. However although we put this on our Impots every year, when I look at the Impots Assessment they send back - they are taxing 'us' on his gross, not net income, so in effect he is paying twice?

In order to make any sense of this, or make a claim that it's incorrect, are we going to have to pay an accountant to sort it out?