Actually, James - having just looked again at the engine in better light, that isn't the hose I need!
On the Rimmers diagram you kindly posted, it is 'kind of' Part no. 9, but the hose that's split is the one that connects to the narrower of the two pipes - the pipe that in the Rimmers diagram doesn't have anything connected to it.
I've found a couple of places I can get that whole Part 9 assembly (from £75 new to around £122) but on my car that whole 'Part' - the metal pipe with smaller pipe running off it and the two hoses - is intact. What I need is the narrower hose that connects to the narrow side-pipe - if that makes sense!
Any help sourcing that would be appreciated. Then I can get out and enjoy whatever soft-top sunshine we've got left!!
Many thanks.