Thanks to all of you who replied.
The diac is split and measures 0.640 ohms in either
direction. marking on it: BTB04 600SAP.
Another similar diac on the board, for the 300w
halogen circuit, is high resistance both ways.
I'll find a replacement diac for the faulty one rather than
experiment with diodes.
There is so little space for diodes anyway.
A low value fusible resistor in the 12v circuit is o/c.
I