Well the garage has not been able to remove the glow plugs because they are tight in and leaving penetrating fluid in the threads overnight did not free them off so they have given up. Fortunately they only charged me for an hours work.
I'm going to check the glow plug resistance myself just to be sure.
They checked that the glow plug relay and it's working correctly as is the correct operation of the ECU. The low pressure was checked in the fuel line and that's OK.
I've checked on YouTube and seen how hard it is to remove stuck glow plugs and most of the videos indicate that stuck plugs usually break on removal so a special extraction kit is required and I'm happy to have a go myself.
Has anyone here done this and how easy is it?