I went to a customers house the other day to fix a rad leak, carried out the repair and topped up the boiler then fired it up ran for while then cut out. When I spoke to idealtech they said it was an air prob and to just keep firing the boiler,no good. Spoke again to ideal and they suggested removing the div valve head to help get rid of the air,no good. The third engineer at ideal suggested removing the pump head to ensure the impeller was still attached to the spindle and it wasn't, put a new pump on and bingo but my question is,why did the boiler not cut out at 0.5 bar as usual to prevent dry firing and does this indicate another component faliure.

Perhaps the dry firing thermistor is duff, or the gauge is blocked and you have more pressure than is shown,

What is the low pressure on these or have I got to get the book out again  

Check resistance using a suitable multimeter
connected across the dry fire thermistor’s
terminal pins
At 25 oC expect 9,700 - 10,300 Ohms
At 60 oC expect 2,400 - 2,600 Ohms
At 85 oC expect 1,000 - 1,100 Ohms

thanks for the help.I think ill just leave it at that