Difficult to select first and reverse

[chris1440]

It's progressively getting harder to select first and reverse.  Sometimes (rare) they slip in perfect, but other times they won't go in at all and I have to put it back in neutral and pump the clutch and try again which usually works.
I've read possibly leaking slave cylinder?  Any other ideas or way to diagnose it? It's a 55 plate with 100k miles