Thanks for the replies folks, I take your point about the chlorine doing it's job, but the reason I have decided to sterilise the tank and pipework this season is because I am only able to get down to the boat once a month.
On returning to the boat, when I run one of the taps I get a really eggy horrible smell for a while untill plenty of water has run through the tap
This is the reason I think the whole system would benifit for a good sterilising??