Is there maybe a tap (on the pipe from the heater to the kitchen tap) that may have been accidentally turned - I had one under the sink that controlled the water rate (been replaced now with a screw type)
Might be a biuld up of limescale in the hot side of tap Norman, If I can get Hold of my mate tonight I will ask him,hes a Plumber, but he might be out on the drink, i'll try though, Ray
Looks as though you're going to have to strip the tap down. Turn the cold water feed to the hot water tank,there is usually a valve above the copper cylinder. Drain down the water. On the tap handle there will be a small grub screw either on the side near the bottom,or under the C on the top of the handle. Remove the handle. You will find a brass nut with the tap spindle running thru the middle of it. Unscrew the nut & the workings of the tap will be seen. the tap washer could need replacing or the workings could be gunked up. Get some WD 40 and spray the workings as you turn the spindle. Hope this helps.
Definitely worth checkin there is no water flow tap on the hot water pipe like postdog says- common in older properties... Could just be that simple. Especially if the cold is flowing fine thru same tap.