Which Single Phase meter should I use together with an Elster A1140 three phase meter, if I want to multidrop them in the same panel.
I want to use this to measure an extra load, like a geyser.
I am currently using a KOCOS modem that clips into the Elster A1140 meter RS232 communications port, and receives power from the meter.

The best choice would be the Elster AS230, with RS232 comms module, since it the installation is relatively simple, using ordinary flat telephone cable (with the RJ clip-in plugs).
It also has the built in capability to switch of the primary load, so if you connect this in series with your geyser, you can use it to switch your geyser on and off remotely, without any additional installation of equipment.
They way you connect that to the Elster A1140, is by using a telephone cable splitter. The splitter plugs into the A1140, and then the KOCOS goes into the first of the two other connectors. The second one goes to the AS230 comms module with a straight flat telephone cable. The same telephone plug plugs into both sides. Care must be taken that enough pins are taken through the splitter to power the modem.