After a quick google of Aeroquip FC598 hose it may be that this hose is designed for a push-fit barb and is perhaps too stiff to be compressed enough by the jubilee clip.
Someone else who knows a bit more about hoses will along soon I’m sure.

As a matter of interest what is your engine spec? Crank, cam etc.
Hi Mike,
The engine is a 1904cc, pretty much rebuilt by Bears Motorsports in Brum/Leicester.
These are notes from when it was fixed (oil plug dropped off on the motorway; dropped its oil; f%^&ed itself) in 2016.
to which I will have said "Let's do the job Properly, as that's the approach we've taken with everything else on the van - no shortcuts".
Parts List:
Ended up getting a new crank case from Autocraft as that proved to be cracked too. Expensive few months! (Auto linea aluminium super case with 10mm head stud case savers, raised roof with clearance for up to 82mm stroke, cylinder bores machined for 90.5mm barrels ,and drilled and tapped for full flow.)
And for the record, this is the engine that it replaced, which we'd had since 2011:
