Hi Danb, I have been doing some reading around this & it looks like the T5 130 (not sure if they all have) auxillery heating which can be modified to work without the engine running.
Modern , direct-injection diesel engines are so efficient they often no longer generate enough heat to warm the vehicles interior. As a result, nearly all diesel vehicles are factory–fitted with an “auxiliary heater” as standard in order to ensure a cosy temperature inside the vehicle.
Comfort in a car does not have to be expensive.
All modern diesel vehicles such as the Volkswagen T5 Transporter/Caravelle that are fitted with a fuel-powered auxiliary heater, can have the heater upgraded to operate as an engine pre-heater— at a very reasonable cost.