Did it improve when you put some grease inside the slave
Not the slave then it should improve if greased under the rubber boot, unless it's a compatibility issueNo, sadly not.
Its just a case of rolling the dice now and hoping its hydraulics rather than the clutch itself.
In some ways finding potentially contaminated fluid has given new hope that it is one of the cylinders. The sound is certainly what would be expected when a slave cylinder goes bad. Subject to the parts reaching me I'll probably fit them end of the week and report back on how it goes.