I have a gravity fed boiler for heating the hot water tank that takes along while to heat up?

I have a 20 year old ideal mexico boiler, which has never gone wrong ,despite the gas board trying to sell me a more efficient one. I was given a quote of �2000 for a new one by an ex british gas engineer but advised to wait till trouble occurred,as I had a good boiler.I have the boiler serviced each year but but the only downside is it takes longer than usual to heat the gravity fed hot water tank.It should take an hour but it is more like 4 to 5 hours.Could this be due to sludge in the pipe or is this normal