replace second hot water cyl with radiator?
 

I have bought a house with evacuated tube collector for hot water. It has one hot water cylinder in the attic and one in the bathroom. Looking the system over, there is a (three way?) valve on the exit pipe from the attic cyl that feeds the bathroom cyl, presumably when the attic tank reaches a certain temp (it has not got hot enough to test this yet). Normally the feed goes down the other side of the bathroom cyl into the hot water system and connected directly to the top of the bathroom cyl. So when the valve changes it will heat the hot water in the bathroom cyl which will then feed out to the already hot water coming from the attic tank. There is no thermostat on the bathroom cyl, BTW

Unfortunately the bathroom hot water cylinder has to go to enlarge the bathroom. It occurs to me that I could replace this with a radiator which would then be heated when the attic cyl reached a certain temp. This would have the hot water supply running thru it.... good or bad idea?

Sounds like the second cylinder in the bathroom is a dump load/safety for the main attic cylinder. In that case a radiator would work as long as it is installed in a place where it can give off its heat.