I love this fruit but they're usually very dear so I never buy them
Tesco had them going at 40P last week so I got a couple.
No idea what to do with it though- they are ROCK hard and no taste at all!
I cut one open,put it in the fridge and it turned all black, but hasn't soften
and the taste's still the same.
Is this just not a good season to buy them?
(suppose that's why they're going so cheap)

If they feel rock hard I would have put them next to a banana for a couple of days which should help them ripen up. Were they Haas or the smooth green skinned variety? That can make a difference to flavour. I'm guessing they were picked at such a stage of ripening that they wouldn't get damaged too much in their travel to the shelves.

PS - If you cut it and put it in the fridge or anywhere else - it will turn black - whether ripe or not.

Thank you Mrs Woof Woof!
It's small and has dark skin,no idea what they're called
Is there a way to make the already-sliced& peeled unripe avocado edible or I'd have to bin it?

Bin them. Never put an avocado in the fridge. This halts the ripening process.

The only avocado with real flavour is Hass, these have green to almost black pebbly rough skin. Smooth skin avocados like Zutano, Shepard etc are virtually tasteless and "watery". They are grown only because they have a season different than Hass and this allows avocados to be on the shelf during most of the year.

If they aren't Hass, I just skip them. Ask your greengrocer for more details.

Thanks for the tips BBQFiend!