Thanks for the replies so far. The system is not mission-critical in that it is my hobby project for my own urban garden irrigation. I have reloaded the software and added a part which will give an obvious LCD display message if it re-starts (I know this won't tell me if it has merely locked up). I also switched off the electric fence energiser (it is to keep cats off the boundary wall).
I thought I had capacitors around the 7809 voltage regulator but buried the circuit diagrams and it's hard to see. I do have protection diodes across the relays, which are switched by small transistors. I thought that was easier/cheaper than using triacs to switch 24vAC for the irrigation solenoids.
I guess it should be straightforward to add a WDT to the software, then with the new restart display at least I'd see easily if it had locked up.