I am applying for French nationality, by way of marriage. We recieved a letter this morning which said we have to sign an attestation sur l'honneur which says we have not separated during our marriage, however we did seperate for a short time. Under French law what will this have any impact on my request for French nationality?
On a strict reading of the law, this is indeed important and can have a huge impact. Uninterrupted “communauté de vie” since the marriage is one of the requirement for citizenship “par déclaration”. That's why you are asked to vouch that you never separated. It's more common to see couples who are still separated or on the verge of divorce having trouble with this requirement but the law does not seem to make a big difference between this and a separation some time in the past, the requirement is simply “uninterrupted communauté de vie”.
I am not a lawyer and I am not aware of any court cases on this particular question (what counts as separation, does it still matter long afterwards, etc.) but the requirement is quite strong. Unfortunately, it could mean that this route to French citizenship is forever closed to you (naturalisation might still work). I would not fill in this form lightly and possibly seek legal advice to evaluate your options.
PS : Note that ”communauté de vie” is not equivalent to cohabitation. You can maintain it while living apart (e.g. for professional reasons) and it's not enough to live under the same roof. It's usually described as the ”affective and material” bound that's expected between spouses.