fluctating temperature in hw mode

I could do with some help plz guys.....Worcester 280RSF, when i run a hot water tap anywhere in house, it runs fine for about a minute, then goes cold, and then back hot again-very frustrating when in shower!!
I've been advised that it COULD be sludge in the sec. heat exchanger??
If this is the cause, is it easy enough to replace, (does it involve draining anything down??)

Does the hot water demand light stay lit or not when the hot taps are on and does the burner stay lit when the hot taps are on or not ????