Can you let me know if there will be any damage done to the water cylinder if the 3 way valve is put in the wrong way round? They will be putting this back the right way round.
What do you mean “wrong way round” ? . A 3 port valve has one input from boiler and two outputs, one to cylinder and other to the radiators. If swapped, it won’t do any harm other than heating your rads instead of the cylinder and vice versa. When calling for both heat and hot water, there will be no difference.
The boilerman fitted it the wrong way, we only discovered it when we went to switch the hot water on and the radiators were getting hot also, the heating comes on upstairs and not downstairs and the water doesn't heat up properly
If there is only one valve, it seems you have more than just a swapped port issue. Perhaps you need to hire a more competent plumber?