ok so had 2 gas safe engineers round as advised by this forum........no joy couldn"t fix it..........i started tinkering about checked pump ,aps,fan,microswitch,........so i removed the pemp sensor but still had electricial connection attached bingo constant hot water boiler doesn"t shut down..............soooooooo screw sensor back in try it again boiler fires up then shuts down stays on shower goes hot n cold boiler shuts down stat calls for heat...........boiler fires up then 20 secs shuts down so would it be a faulty temp sensor read 10k when cold....... or couldit be faulty safety thermostat..........please anyone i"ve gone by the rules and got 2 gas safe engineers in to no avail