Glow plug / starting problem

Hi,

Strange problem, Any ideas very welcome.

Octavia one 1.9 tdi 2002 120000 miles been fine for 10 years.

Not starting well when cold. Glow plugs checked and changed, temp sensor changed 103 relay checked and working fine. Fine for rest of day after first start.

Ignition then we wait for a few seconds to start, has always started first turn even in very cold. The plugs are now not coming on until the engine starts. Then 12+ volts at connector, when disconnected from plugs. Why not on when waiting for heat? Clearly the wiring must be fine. Got me stumped, don’t really want to put the plugs on a new circuit but that is an option.