Thanks for the info Ben. I am not too worried about the inheritance taxes that will need to be paid - I won't be here!!
To avoid any legal problems a will should, according to the "The Hague Convention" of 1961 in general satisfy one of the following conditions:
- valid within the laws of the country in which the legator was living,
- valid within the laws of the country which gave the legator his/her nationality
-valid according to the rules of the country in which the legator has made up his last will
When the will specifically and exclusively concerns houses etc, the last will should be valid according to the laws of the country in which the estate is located.
If the last will will be executed in the UK you need to include the French property in it. If you live and hope to die in France, better to go to a French notary and have it registered according to the French rules. But neither a UK will nor a combination of a French and UK will can exonerate your inheritors from the French inherence taxes on goods and properties located in France.