Central Heating Problem

Hi guys wonder if anyone can help.

I have central heating (not split system) at home have had it for around 5 years and have had no problems at all with it until today. Whilst standing under the vent I felt the air being blown out was cool (usually it starts cold then heats up after a few mins obviously) but after a while it was still blowing down cool air.

I went to the thermostat and put the temp up and the warm air came out as usual so I thought it was being a little tempremental... when it reaches a certain temp it should cut out like normal but now when it reaches the right temp the heat stops but my vents are still constantly blowing out cool air even when you set to off. So its still heating correctly but just not cutting out and when the heat stops Im just getting a constant cool air blowing through.

Anyone know what the problem could be ?