Took a gamble and bought a pair of 700 x 40 Gravel Roads for the 2012 Croix de fer.
And they fit!
I'd heard that the biggest tyre it would take was a 38mm.
Plenty of clearance on the rear and about 5mm under the fork crowns.
Always been nervous bikepacking on 35mm tyres, given that my weight fluctuates between 95 and 102kg, plus the carry weight. Bit more (tyre) volume should help now...

On my second set now, great tyres