Halstead ACE HE 24 Locks out unless the front panel removed.

[Agile]

this type of combi does not have a venturi
have you checked for EARTH CONTINUITY, POLARITY and RESISTANCE TO EARTH.
what is the gas supply pressure at the inlet pressure test point
what are the CO2 values and CO/CO2 ratio
when was it last serviced

Do you really think this is appropriate advice to a DIYer who seemed hell bent on doing it himself.

Apart from that, its against the forum rules to give advice on DIY gas work or to encourage gas DIY.

The only correct advice is to get an RGI.