We fixed ours by getting a new Worcester boiler... The Ferroli was rubbish and I would not recommend it. The new boiler provides abundant hot water with no temperature fluctuation at all.
Apologies for the long post, and thanks in advance for reading it!
Problem: Shower water temperature drops to lukewarm, then returns to hot on 1-2 min cycle. The problem is pronounced in winter, presumably because incoming water is colder. Almost unnoticeable in summer.
This has been...