Hot Water Problem with Ideal Isar He24 Combi?

Hi, Can anyone help?

A friend of mine is having problems with her hot water supply from her Ideal Isar He24 combi boiler.
When you draw of hot water the boiler is kicking in and initially the water is very hot, but it then quickly drops to being luke warm. If you adjust the flow at the draw off tap you can get the water to become hot again but only at a very slow flow rate. This gives a problem when using the shower as you can't get a decent flow of water through without it being cold...
I have read a few blogs suggesting that cold showers are a one disadvantage of combi boilers but this problem has only just come about since Christmas, previously it was fine.
The CH works fine, although it is very rarely used. The apartment was empty for a couple of weeks over Christmas with the heating timed to come on intermittently which was the first time it had been used for a while.
Having read a few similar blogs, I thought it might be the diverter valve. My thinking being that if it hadn't been used for a while to divert to the CH it may have become stuck, as there would have been no DHW draw off during this time.
So I removed the actuator head and checked the travel of the valve mechanism push rod. There seemed plenty of travel circa 10mm but I manually operated it a few times to make sure it was operating freely. I then replaced the actuator and checked the system again, no change.
The valve seems to be correctly working and diverting the water according to the demand (DHW on/off, CH on/off, both on/off) it’s just the temperature.
The only other things I can think of are;
a) the diverter valve may be blocked, so although working correctly, the flow of water through it is restricted and therefore reduced?
b) The heat exchanger plate is blocked again reducing the flow of water through the plate and limiting the hot water output?
c) The core temperature of the water has been reduced by the cold weather and the heat exchanger can’t cope at full flow. (If this was the case, I don’t understand why it runs hot initially)?

Please can anyone shed any light on what they think the problem may be (and how to resolve it)??