Thanks @Murdochcat...
Both the original and replacement fans are quite substantial (maybe 'inline' is the wrong descriptor - see attached, mine is model 33700) and weren't that cheap (£150-200-ish in 2019 iirc). Not sure if bigger domestic duty fans are available, but which brands should I be...
A few years ago we had our bathroom refitted, which included a new HIB Cyclone illuminated inline extractor fan above the over-bath shower...
The fan worked well for a while but a couple of months ago the fan started to get noisier and way less effective: eventually we could hear it struggling...