These problems may not be related...

I have a Worcester Bosch Greenstar boiler (an early condensing combi)
about 6 years old. For some months the hot water has taking about 3-4
minutes to come out hot, as if the pre-heated store isn't working.
Timeswitch is set correctly, the boiler is not in ECO mode.

Also, in the last week the rads have been taking a long time to get
warm. The rads upstairs get hot first while the downstairs ones are
still cold. The burner comes on for about 3 minutes or so then off for
3 minutes, so the whole house is taking hours to heat up. There is a
clunking noise coming from somewhere inside the boiler.

Any ideas? Pump? Sludge in the pipes - the central heating system is
also 6 years old. Whihc should I check first?

Many thanks!