Was it really 'major maintenance' at Cardiff?
The only reason I ask is that when my 40 year old aircraft type (which are all HMFC can afford) receives major maintenance at 'Scrapheap Challenge' just down the road from Rhoose, they're always given a very thorough Full Air Test before going back into service.
Mind you, flying those things from an aerodrome with narrow taxiways and only one 6000' runway after a total rebuild concentrates the mind somewhat.
4 hydraulic systems? Luxury!!
Sounds like the BA crew, not unsurprisingly, did the correct thing and yes, it would certainly require ATC to be informed if you were landing with a single hydraulic system fsilure.