I replaced it with some simple electronice, when the wood fire is hot
the gas boiler switches off, and all the zone valves open. When the
wood fire is cold, the zone value closes it off.

The problem is that in an electrical or zone valve failure, the system may
explode. Are you sure that in the event of valve or electrical failure that:

(a) There is a direct unvalved vent path from both appliances to the
outside.
(b) There is a direct unvalved water feed path from the header tank to the
appliances that doesn't share with the vent path.
(c) For any solid fuel appliances, there is a direct unvalved flow path from
flow to return to dump unwanted heat.

Christian.