Quite simple really, glow plugs are not getting hot. Its a simple circuit, the temp sensor tells the ecu its cold, the ecu energises the glow plug relay for the correct duration, the glow plug light goes out, then you crank. but its taking a few seconds and chugging at less then half the idle speed before I hear a relay click and it idles perfectly.

I have new glow plugs fitted, when you disconnect the temp sensor, the ecu defaults to maximum plug duration 20 seconds, on the multimeter the buz bar seems to be getting 8v then it slowly raises to 11.7v just before the 20 secs is up. I have pluged in a spare plug and keyed on with the temp sensor disconnected, the glow plug light comes on but the glow plug fails to get hot.

I have replaced the main engine earth.

I have also tested the glow plug relay on a battery with a continuity tester on the switched side at it energises ok and switches, but is does need to shunt 35amps or so

all I can think to do is replace (£17!) the plug relay that appears to function on the off chance its latching but not shunting enough current,

Any ideas what I can try next!

where are you earthing your tester?the engine has a lead that earths to the body/battery as well,check that for con

is the battery is in good condition?