With the help of Ian I have a better understanding in the case of Wireless Witchcraft versus Model Railways.
The default settings for the Lenz Lan/Xpressnet interface are:-
* fixed IP;
* IP address of 192.168.0.200;
* subnet mask of 255.255.255.0.
This information comes from a manual which can be downloaded from a Lenz website... and is not present in the paperwork which comes with the product. The download manual states that the IP settings can be changed by use of a PC connected to the device - a method which Simon found to be diabolical.
The Touch cab software on the i-pad uses the tplink device to connect to the Lenz interface... and needs to be configured with the IP address of the Lenz interface (IP address as above).
Finally, Ian advises that the connectivity shown in Simon's original sketch, in the first post, can be met by configuring the tplink box as an "Access Point".
So overall looks as if we had a configuration error when setting up the bits and pieces last week.
regards, Graham