Heatslave 15/19 lockout?

Hi I have a worcester heatslave 15/19 have replaced the expansion vessel after noticed it was slightly leaking and getting lockout on boiler, think there may have been another problem as well as boiler keeps locking out, i reset it but keeps locking out. Will only run for about 5 or so mins at the most or if it does manage to heat up will lock out again if you turn shower/hot water on for any length of time. cheers