A calorifier tank is like a mini hot water tank from your airing cupboard,.. the L5 would sit on a separate circuit with a separate pump controlled by a thermostat on the tank,... when the tank starts to cool the thermostat switches on the pump and the water flow automatically starts the L5 so the tank is always kept warm.
Your WFP pump goes into the other inlet in the bottom of the tank and the outlet to your reel is on the top, feeding cold water in the bottom which is heated as it moves up the tank and is hot by the time its on its way to your reel and hose.
The downside is that you have to heat the tank before you can get hot water to your brush,.. it'll prob be 10 - 12 minutes at the beginning of the day before anything hot gets to the brush head. The other downside is price,.. calorifier tanks usually cost about £200.