hi
i have just built up my steel de rosa endurace ,it is a good looking fine handling frame ,a good bike but with a serious problem regarding tyre /chainstay clearence only 1mm with a 23c tyre i can use 20mm tyres but that makes the ride too harsh . i wood like to know anyones thoughts on this.

I have DeRosa Team frame and now ride on Michelin Megamium2 (23mm). Megamium is very fat tyre and I think it is more 25mm than 23mm as stated. With those tyres there is some 4mm between tyre and seatstay, and there is a plenty of space to the chainstay. So there is no tyre/chainstay clearence issue on my frame.
Is tyre 1mm far from both stays....maybe you have an dished wheel that has gone to one side...probbably left?

your frame sounds ok , my other frames have the same amount of clearence ,maybe this frame was built wrong or built only for very narrow tyres ,either way it will probably end up on a wall.