The realisation of a disconnected duct prompted a full investigation into the system, starting with the easy to access visible room terminals using a Depstech borescope. The borescope clearly showed that all the connections at the terminals were good but was inconclusive in determining if there was any issue at the other end with the manifold.
However, the borescope did clearly show significant build up of dust on the EXTRACT ducts, but the SUPPLY ducts were clean and the kitchen extract was only slightly dusty (I assume due to the kithcen greese filter). The ensuite had far less dust build up than the bathroom and as both have a similar air flow rate, I suspect that as the ensuite always had a black fleece filter fitted suggests these filters at the terminal are effective (as one would expect).
So after 5 years my extract ducts definitely need cleaning and arguably should have been cleaned earlier, so a 3 year maintenance interval looks appropriate. It would be reasonable to assume that there will likely be a multitude of ventilation systems across the UK with clogged ductwork and form my limited experience there doesn’t seem to be an awareness of this issue. The question then is how to clean the ductwork. Zehnder have just the kit but at over £1,600 it’s not a DIY product and there doesn’t seem to be an option to hire the equipment or specialist contractors to undertake this work. (Here’s a video showing what’s involved – view from 36min 40 seconds)