I'm intending to drive to Turkey through EU countries ie; France, Belgium, Germany, Austria, Hungary, Romania and Bulgaria. I know I'm ok as a UK national, but the wife is a Turkish citizen with indefinite leave to remain in the UK. Could she use a multiple entry French Schengen visa all the way to Turkey and back?
Yes, you can enter all those countries, including Bulgaria and Romania with a valid multiple-entry Schengen visa. Even some non-EU countries might grant your wife a visa exemption based on her Schengen visa (if she needs a visa in those countries in the first place). See Which are the non Schengen countries that allow entry for Schengen visa holders? for details. Alternatively, getting a visa for Romania and Bulgaria should be quick and easy for members of the family of an EU citizen.