I'm beginning to think that I may just be low on transmission fluid. I
noticed little or no problem until I had the oil changed, and I think
maybe one of the guys let out some transmission fluid instead of oil,
the drain plugs for them are close together and easy to confuse.

I don't think it's the clutch on the grounds that I can shift freely
while in park and every now and again while driving. Also, I don't
hear any grinding noises while shifting.

It might be temperature related, it's been very cold the past few days,
but it strikes me that if the shifter sticks every time it gets cold
that the previous owner would have noticed and done something. Plus,
the shifting doesn't improve even after the car is well warmed up.

Thanks for your help so far, probably going to change the transmission
fluid to Redline, but any final thoughts would be most appreciated.