By chance I happened to be in the Boucher Road area today ( not something I experience often, thankfully ) and popped in to Calumet.
Had a 'feel' of the Canon 400mm 5.6 prime, lovely piece of kit and very portable but unfortunately they don't hire that one out.
One lens they do hire is the 300mm 2.8; incredible piece of metal and glass which comes in its own carrying case. Very tempted, for 60 quid per day, to give it a try.. no way I'm ever going to own one!