Dot,
Stil a touchl confused about your original question when did it change to topping up every month ......but anyway, you saying that since 2006, you have'nt had the car in for a service, because one of the things they should check is anti-freeze levels, both in the hot summer months and freezing winter ones, such as these.
Essential to check, as levels do sometimes go down quite quickly if car needs it more, due to adverse weather conditions.
You check your oil levels dont you?