Based on the information you've provided, it sounds like the SCR (Single Channel Receiver) unit for your Drayton Digistat heating system may be at fault. The lack of lights showing on the unit, as well as no response when changing the temperature on the wireless thermostat, are both indications that the SCR unit may be malfunctioning.
If you have confirmed that there is 240V across the N and L terminals, it is safe to assume that the unit itself is at fault. The red LED light should light up when the unit is mounted, so the fact that it is not lighting up is another indication that the unit is not functioning properly.
It's possible that the problem could be caused by a faulty capacitor, as you've mentioned. However, since the unit is not lighting up at all, it's also possible that the problem could be caused by a different issue.
If you do need to replace the SCR unit, you can try searching for it online to find the cheapest place to purchase one. However, you should make sure that the replacement unit is compatible with your specific Drayton Digistat system. It's also important to check the warranty of your system, if the SCR unit is still covered by it, contact the manufacturer or supplier.
It's also possible to use non-branded equivalent of the SCR unit as long as it is compatible with your specific Drayton Digistat system although it's impossible to give you a specific brand name of non-branded equivalent of the SCR unit that is compatible with Drayton Digistat as it may change over time.
As a DIYer, you can take a few additional steps to further diagnose the issue with your Drayton Digistat SCR receiver:
- Check for loose connections: Make sure that all of the connections to the SCR unit are secure and tight, including the power and wires connecting to the thermostat.
- Check the thermostat: Make sure that the thermostat is working properly and that the batteries are not dead or low. You can try replacing the batteries and see if it solves the problem.
- Check for power: Use a multimeter to check for power at the terminals of the SCR unit, including the N and L terminals as you have already done.
- Check for continuity: Use a multimeter to check for continuity between the terminals of the SCR unit, this will help you to identify if there is a short circuit or a break in the circuit.
- Check the wiring: Inspect the wiring of the SCR unit and make sure that it is not damaged or corroded, if it is, then it may need to be replaced.
It's important to remember that electrical work can be dangerous and if you're not comfortable with handling it yourself, it's best to consult a professional electrician.