red cut off light keeps coming on. ?

i have a halstead finest combination boiler found out it is 10 years old moved into flat 3 years ago no problems had it serviced the first year all ok then it started the red light would come on when filling bath in morning then a few times at night when the heating was on all i could hear was the pump running red light showing.had it looked at the rads got bled and the sysytem got topped up was told low pressure all was fine until now have bled rads again and my gauge stays at 1.5 in morning runs for 5mins then cuts off the other eve when getting home from work turned heating on took 2 attemps then lit into action rads started to heat nicely then red light came on. can anyone tell me what is wrong just want it fixed as starting to get pretty cold at night now.