Smell from Alde heating system

We have a Sterling Eccles Quartz with an Alde Central heating and hot water system. We have not had any problems prior to this trip, but now we have a bad smell coming from round the Alde boiler. We can't decide if it is a stagnant water smell or if it is from the Calor gas.

Have drained and refilled water system and turned off gas supply, but we still have the smell.

Any ideas or suggestions?