worcester heat slave 20/25 oil smell?

hi we have a worcester heat slave 20/25 a few years old now but we have terrible oil smell (worse when boiler is switched off during the night
boiler repair man can find no problems ?
smell from bolier when (switched off over night) is terrible gases out the whole cottage their are no leaks from pipe work or bolier inside is bone dry?
boiler works fine during day but smell gets betterwhen boiler is used all was fine until we fill oil tank up?
please help as cost of boiler man is expensive and no cure .. their are no signs of oil leak at all .
thankyou please help us ..