if your heating pipes arnt getting hot from under the boiler then it either blocked heat exchanger or hot water thermistor is on the blink (normally around £6) and takes 5 mins to fit, worth a try first
Change hot water thermistor (in pipe)
Turn boilers isolation switch off
turn water off
open hot tap where spout is lower then the boiler to drain the hot water out the boiler
find where the domestic hot water thermistor it located
take the two wires off it
with either a small socket set or spanner unscrew slowly and now you will find hot water tap starts to run a bit quicker
replace thermistor and repeat in reverse
Change hot water thermistor (clip on)
Turn boilers isolation switch off
find where the domestic hot water thermistor it located
take the wires of and unclip
clip new thermistor on
turn boiler back on
Simples
Thanks for all replies. Do I need to drain central heating in order to clean plate heat exchanger?
i would strongly suggest that you do NOT touch a 24cdi secondary heat exchanger unless you know what you are doing because they normally leak like a siv after