Looks like it’s available on Interrail’s own booking service for €19.

https://www.interrail.eu/en/book-reservations#/

Thank you very much Ralderton, just booked them.

For future reference,  is Interrail's own website the only way to book seat reservations French trains with compulsory seat reservations? 

Thanks again 

No, it’s usually the worst place! Often has no availability, and comes with a €2 fee (that reservation is €17 really, plus a fee), and doesn’t allow you to choose your seats. There are better ways for most reservations.

But in some cases it can be useful, and seems to be able to easily book the odd thing that you can't find elsewhere.

Agree indeed, I was trying to avoid using the interrail website too, but couldnt find any workable alternative in this instance... 

Interrail website is still the only online possibility to book this train. You can also call SNCF to book. No booking fee and they will also send it by email.

Thanks,Rvdborgt, great to know for sure, thank goodness for this forum !