Jammed gate valve?

The gate valve on the feed from my cold water tank to my hot water cylinder is stuck firmly 'open' - as I discovered when I needed to shut it in an emergency :-(

All attempts to shift it have failed. It is located right behind the tank in a very tight space with virtually no room to swing a wrench on it, tho' I have tried hard. I have even considered replacing it with a new one, but having drained down the syetm found that (a) I could not get enought grip to undo the compression nuts (the limited space for wrench again) and (b) the new gate valve seems shorter than the old and therefore the pipework probably will not align properly anyway.

Can anyone suggest any way to free up the old valve, or will I just have to call in a plumber ???