A fault codes reader would be an easy next step.
See if it can talk to the ecu. (i.e does the ecu have power)
See if there is an imboloiser code stored or anything else interesting
Problem solved but cause remains a mystery. after al All I tried, with everybody’s help, I decided to just start tracing all the circuits from starter back to find where I was dropping power Started at the starter, worked back through the ecu, on towards the ignition switch. Followed back from the body processor module towards the neutral switch finding all good. Found power to the engine control module. Checked power to the relay there finding good. Checked relay, seemed alright but just as a thought, switched out the relay with the horn relay and, low and behold, car started right up. Horn even worked. Still confused as to why taking the battery out and back in would cause this. Thanks to all for the help and suggestions. Im stll going to get a spare relay to have on hand.
I’m going with slightly corroded contacts in the start relay not allowing sufficient current to pass to the starter solenoid, but having sufficient to power the horns. OR just taking the relay out and re-inserting could have produced the same result?
why taking the battery out and back in would cause this
I think it’s probably just an unfortunate coincidence.