My E2200 panel van overheats when climbing hills and most 5th gear driving. Temp is stable in town.
I have had the radiator out and is OK. Water pump is OK.
Think it may be to do with the fan behind the radiator. Is this electrically powered ? if so where is the switch/wiring to check this.

Any help would be very much appreciated.

Don't trust radiator shops. Good flow does not mean 4 or 5 tubes are'nt clogged. The only way to be sure is remove the tanks and rod the tubes clean. Most rad shops are too lazy to do this. No fan is needed on highway driving with a 60mph wind across the radiator. Sounds like classic radiator symptoms to me.