Sime super 90 plus problems?

Hi, We have a super Sime 90 plus boiler and about a year ago we had to change the dreaded diaphram in the diverter valve as we lost hot water but still had central heating. The diverter valve IS working properly now and operating the micro switch.
Our problem now is, the temp of the water has reduced dramatically, when we run a bath it is only luke warm.
We reduced the water pressure into the boiler via the main stop cock, and we now have piping hot water, but now takes hours to fill the bath.
Our thinking behind it is that the heat exchanger could be furred or scaled up and thus not enough water is being heated by the heat exchanger.
We are however a little confused as we were told there were two circuits in the heat exchanger, one for HW & one for CH, but there is only one circuit running through it.
The CH is piping hot and working fine.
There is a cylinder at the back of the boiler, to the left of the diverter valve? what is it for?

Any help would be greatly appreciated.