Dodgy clutch, any ideas?

Evening chaps,

Looking for a bit of technical expertise.
My Volvo XC60 has been showing a weird issue and I am at a loss to work it out.
About 6 months ago I took it on a long journey, about 250m, mostly motorways. Near the end I noticed the clutch pedal had gone soft and stuck to the floor. I also could not change gear. After pressing it a few times, it reverted back to normal. While driving around town, all is OK. On the return journey the same thing happened, but again depressing a few times makes it mostly better again. It's not losing any fluid, but must have some air in there I guess.
Any idea on what the fault could be?
It has an internal slave cylinder, so difficult to look at it without dismantling a lot of car.
Ta,
R2R

2013, 100,000 miles, I believe its an old Ford engine in this model. 2.0 D4.

I had this on a Focus ST, mine wouldn't fully pop back up if I hadn't used the car for a while, it's like the pedal mechanism dried out and needed a few pumps to get it freed off. Had the car 5 years, whenever I'd not used the car in a few days, I'd pop my foot behind the pedal to pop it back up, it'd be fine until I'd not used it much again.

Nothing to do with the clutch itself, it's the pedal. After 2 years (a remap and more pooower) I went for a RS Clutch, pedal still did the same thing.

I suspect it could be this, but it's an expensive job, especially as doing the clutch plates at the same time makes sense.
If I only drive mine round town, it's fine. Long journeys, it isn't. I dd bleed it a while back and it had loads of air in it.

sounds more like a master cylinder to me, shouldnt be that big a job, although some are awkward.