Thanks for the pointers,
Is it possible to remove both cables from the gearbox without damaging the connections?
My think is if I can separate the cables and gearbox it will prove the cables are at fault and I can also hang the cables up and drain oil down them.
If you spray WD40 or similar at the ball ends, then I use a 13mm open ended spanner between the lever and ball to gentle prise them apart.
Incidentally modern cables are nylon/teflon lined I think , so pouring oil down them is unlikely to make much of an improvement.
I recently fitted new cables on a 2010 Doblo 1.6 MJ six speed, it involved car on stands, ideally battery and case out, centre consol and plastic around gear lever etc. heat shield from exhaust off, plastic securing clips that hold the cables to under floor and the bolts that hold the rubber plate to seal cables to below gear change are of floor.
Incidentally I had hoped it would allow easier engagement of first gear, it didn't.

Every other gear works fine , but you have to feel it into first to allow full engagement by lifting the clutch very slightly.
If you ram it in it doesn't go fully in, but gently as I said works with no other issues.
New pattern cables were around £100 approx.
By the way the only adjustment is under gear lever on the cable that controls side to side movement.