This evening I finished swapping the faulty transformer for the one from my donor set.
There were a couple of differences in the connections. The original transformer was connected to pins 2 and 7 on V4, the 6V6 output valve. On the donor set, these connections were to V3 pin 3 and to chassis. I re-connected to the 6V6 with some difficulty as the "new" leads were not as long as the original ones.
On my original transformer, the two wires on the other side were both (very badly), connected to the 230v-250v connection on the voltage selector.
The donor transformer had one connection to the 200v-220v side and the other to the 230v-250v side. When I fitted the donor transformer, I copied these connections, to both sides on the voltage selector.
Tomorrow I will repeat all the testing and fingers crossed, nothing will emit smoke this time.
Gus.