A minute of Hot water ONLY...

hi,i dont know what is wrong.Dont know if the boiler or the shower.At first i dont have any hot water,then
Last week the boiler was flashing a sequence of 5 red lights,so the plumber came changed both sensors.
But after that i am still getting a minute or two hot water.I can hear sound from the boiler after 8PM,then i go shower around 9pm,
and i only get a minute or two hot water then cold then warm for 10 seconds then Cold.But when i turn off the shower and wait for a while and then turn on the tap i get little hot water again then cold.Please help.fed up with showering in cold water........

the next best thing to change is the pump control board

Check the pump and PCB. These could be the most likely causes. Also check the water level in your header tank which could be low.