As above really, I have had 35c conti cyclocross tyres which came stock with the bike, which fit ok. 17mm internal rims.

Fit in my croix de fer 30 (2015) in fact I have squeezed a 45mm in the front and run the 40 nano at back.

40 nanos measure as 38mm at 50 psi IME.

Thanks all, will have a measure and give them a go.

The answer to this is they fit fine in the frame, but are to tall for the fork, and rub on the very top, even at low pressures, on the plus side, the bike looks like an absolute beast! Any recommendations for a fork that has a bit more clearance?

2015 (onwards) CDF forks had much more vertical clearance to address this issue. Available on ebay in carbon or steel for £150