Having had the mains power off to install new socket and light on swithing poer back on alarm now in lockout. No response to key pad (no bleeps). Key pad illuminated, power illuminated, Day illuminated, Zone 1 illuminated.

How do I get out of lockout?

The battery in the main panel is flat, often this damages the charging cct on these panels, however not in every case, power up the panel with the battery removed, close the lid on the panel and see if the system responds, if so replace the battery with the same you took out, an electronics shop such as maplin will have one. Taking the battery out is not the long term solution but will give you an idea if the panel still works.

Really it's worth having an engineer out to check the charging cct on the panel and give the system a once over, sometimes replacing the battery after a power failure where the system has been left with a battery flat for so long can cause real problems.

When was the alarm last serviced?