Sounds like you'd get a lot of side spray. 45mm guards for 38 mm tyres sounds more like it. Length wise you'd be ok as the wheel is smaller than what the guards are designed for.
Thanks both - annoyingly I can't fit 45mm guards in my frame I don't think so I might just go for 28mm tyres and stick with 700c and let mudguards do their job.
Sounds like you'd get a lot of side spray. 45mm guards for 38 mm tyres sounds more like it. Length wise you'd be ok as the wheel is smaller than what the guards are designed for.