I don't tend to do Eurosec and certainly never done a CPX.
I don't get how its only become a problem after 10 years?
It naughty really to lock a panel out, especially if a customer has paid for and owns the panel.
okay now we know what you have got, if the panel is locked you can get the engineer back if they are still trading?
Whether you take advice on whether they had the right to lock it is another matter and what you get charged should be no more than his usual call out fee or service.
If the engineer is available but doesn't want to resolve the lock out speak to CAB on where you go next.
if the engineer has gone to all that effort to lock you out, you probably wont be able to safely default the panel and regain control.