Just had my old steel one replaced for the new insulated energy efficient type at a very reasonable cost.After a while they do rust and also if it has an immersion fitted they corrode.Its worth checking to see if any water is leaking onto the roof,if so its a new one needed and have the immersion checked at the same time.
All Aspects on BMB are excellent,reasonably priced ,quick,guaranteed work,ask for Selcuk