Triangle Tube Challenger never stops heating?

We've got a Triangle Tube Challenger combi boiler & water heater, and now that it's getting warmer and the thermostat has shut off, I've noticed that our radiators are still warm. Is there any way to turn off the TT's heating entirely?

In case it's any help, I've also noticed that the boiler cranks up every two minutes, displaying 1, 5, 8, 1 in sequence.

thanks in advance!