Is there an ideal topology for a z-wave network?

[garethhowell]

I'm having a lot of problems with my z-wave network. I have an S2 controller in the house connected to two switched AC outlets (and thus permanently powered) and two battery powered devices. All work flawlessly. The problem is in my office at the bottom of a 7m garden.
Both of the powered switches have line of site to another two powered switches in the (wooden frame) office. These then connected to a battery powered thermostat.

This should be capable of forming a resilient mesh network between the powered switches, but it isn't. The devices in the office regularly disappear after many weeks operating normally. They end up being marked as dead (although Domoticz still logs updates from the power meter in each switch).

I've tried different ways of adding the office switches, but the resulting network (as shown by the S2) is never a full mesh. The office switches can't be added using the include function in Domoticz because they are out of range, but I've tried adding them by taking the S2 down the office and using the S2 button. I've also tried adding by taking the switches into the house, adding them and then moving them down the office and healing the network.

Nothing seems to be reliable. Is there anything I can do to improve matters?