Shower cycling between hot and cold

Combi Boiler: Worcester Junior 24i
When taking a shower after a minute or two the water cycles between hot and cold every several seconds and then after a minute or so it settles down and I get a steady flow of hot water again (only happens when using the shower). Having read some of the threads on this forum could this be the plate to plate heat exchanger? I've looked at the schematics for this boiler and there is another heat exchanger but seems to be encased in a cylinder.
What is the difference between the 2 heat exchangers and which one is likely to be causing the problem? If limescale is the issue how could this be causing the water to cycle between hot and cold before settling down?
Incidentally this seems to be happening every year so any ideas on how to avoid this in the future would be appreciated

 

Does the boiler shut off then startup again at the same frequency?

 

Sorry absolutely no idea. We're in a bungalow with, you guessed it, the boiler in the attic!!!

I will check for this as soon as and get back to you

 

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