Hi I have the webasto Thermo 90st fitted but only has the on off control button with no thermostat control can you recommend the upgrade timer / thermostat I should req for this and how long is your extension cable that allows me to do it ( on a boat )

Webasto do not recommend installed a room temp thermostat to control their water heaters (primarily due to short-cycling) It is possible to upgrade your on/off switch to a programmable timer however, yes. Perhaps email a photo of your electrical connections at the switch, this would allow us to advise exactly how the connection would be made