24 CDI RSF low hot water pressure

Hi guys
Wonder if anyone could point me in the right direction .
I've got a 2002 Worcester Bosch 24 CDI Combi boiler. Great machine, but the hot water output has been gradually getting slower over the years, and now takes 20 minutes to fill the bath !. Temprature is absolutely fine, and mains pressure goes into the boiler (via a Cistermiser), so I'm stumped. Thought it could be furred up pipes, but the Cistermiser is supposed to stop this
Any words of wisdom gratefully received (and a pint if your near Newbury ! )
Cheers
Bazzer :

What is the max flow rate on HW? Do you mean you have to half close a tap to get it hot enough for a bath?

Hi Chris
No, I open the hot tap to the bath fully, its just the water comes out quite slowly. Its easily hot enough (on position 2 out of 4 on the boiler), but when we first had it the bath only tok about 10 minutes to fill
I'm still stumped
Cheers
Bazzer

Two possibiities (there may be more!)
1) Limescale. Can be cleared using a pump, by eg circulating descaler via washing machine taps
2) Flow restrictor has come to bits or got blocked - would need dismantling
3) Not just the bath tap is it? There may always be more, in plumbing...