Hot water brown when Central Heating is switched on?

It sounds like you have a combined boiler system that both heats your domestic water and supplies water to the rads. The water sits all year in the heating pipes and gets fouls, then when heating season comes it circulates throughout the rest of the system. You probably need to flush the system to get all of the old stagnant water out. There should be a drain valve where the water comes back to the boiler from the last rad. I would open the valve and let it run a while, until the water runs clear.