Are you asking if mains cold water is connected into the hot water pipework ,then how does the cold water get up into the loft tank ?
i dont no if its stuck or not. i think ive got a airlock on my hot water after changing a tap. its been suggested it could be the gate valve when i isolated the water. ive tried blowing the p-ipe out with the cold tap but it hasnt worked. The gate valve is about 6 month old and doent feel like its got anything wrong with it.
No ,your wrong again !! Mains cold water introduced into the hot water pipework would travel into the distribution pipe at the top of the cylinder and therefore up the expansion pipe into the loft tank.
That's odd. Try again as xednim said above. You should get water ,albeit cold ,coming out of hot taps before the loft tank starts to spill out.
OK ,unlikely that you have an air lock ,but try this to rule it out completely... Connect cold mains into hot pipework. Close all hot taps except one that is the lowest in the house( if kitchen hot tap is not connected and no other hot tap on a low level,then open one anywhere).Run cold water until it comes out the open hot tap ,then close it ,and open another hot tap. When water flows from that one ,repeat until all hot taps are giving cold water. Then turn off the cold mains ,and put things back to " normal".( in other words whatever you did to allow you to connect cold mains into hot pipework , put back to original state). If you now open all hot taps ,and they work fine ,you had an airlock. I suspect they will all drain out after a few minutes and then you will get nothing . That means your gate valve is stuck closed.