hi I;ve just managed to do mine online but I had to phone the tax office in Pezenas for help as I couldn't seem to find the avis. So here goes...Once you have your code d'activation through the post you go onto impots.gouv.fr and follow the instructions to add your bank account. All well and good so you then try to pay but you need the avis d'impostion which you can't get to from here. So you have to go back to the beginning impots.gouv.fr log in to your espace professional. Email & Passwod.
You may get this message Nous vous prions de nous excuser pour les difficultés actuellement rencontrées : in red at top of screen, ignore it if you can see yor name and siren under dossier courant on the right click on Then click on compte fiscal.
It takes you to next page, click access par impot (first box on left - gives u a drop down of CEontribution Economique Terrotorial - click and it opens to right again - select cotisation foncieres des entreprises, next again to the right and select avis d'imposition. Note the number down.
Click on annee 2015 then click on Nature d'Avis and it will open the D'avis (a familiar looking form will open up and you can print this off) There is a blue button at the top for payer - click this and it will ask for your email address twice, tick to confirm you wish to pay.
Make sure you confirm all the way through until you get the message saying Votre ordre de paiement a bien été enregistré le xx/11/2015. After you will receive an email confirming payment will be taken automatically (if that's what you opted for)
Phew - mine was 163 euro. A relief as I was expecting it to be around 300.