nails for 22mm t&g floorboards

Don't worry about their length ... just make sure they have a barbed profile to prevent them pulling out and causing squeaky floorboards.
Better still, use some small head screws screwed at an angle through the tongue into the joists. That way, they'll never squeak and you won't even have any holes to fill.

 

As roger says use some screws, you don't want squeeky boards.
Much better than nails. As in time they work loose.
Builder isn't living there, you are.

 

Thats only 20mm into joists, but would do, 50mm preferred.

 

Lost head nails are no good for your floorboards as they have no actual head on them to talk about.
You have t&g boards, therefore you you want to be secret nailing the boards and not face nailing them, 40mm and 50mm screws are also no use for fixing through the tongue.

You could hire a porta nailer to nail the boards down, but also cramp them before nailing to get them tight, even although the nailer helps with this.

You could also use cut nails, which are the traditional method or wire nails, but the best method for you when not experienced, would be the nailer to stop you damaging the board with the hammer above the tongue, which is easily done.

 

I would certainly use screws instead of nails. I'd use something like these, allowing at least 30mm into the joist. https://www.ebay.co.uk/itm/LOST-TIT...hash=item25bed26f15:m:m0QMAj27odHV43r631fJ1SA

 

Yes, it would be difficult, as you would have to lift all the boards on the tongue side right the way to the side of the room. That would be the case for any 'hidden' method.
Since it's going to be carpeted anyway, why bother with a 'hidden' system? Why not just face screw the boards? Then, if ever you need to get to pipes, all you would need to do is cut the tongue off one of the boards and unscrew it. Once the now-tongueless board is out, all the other boards behind it can be lifted simply by unscrewing the face screws.

 

Yes, just drive it at an angle from the foot of the tongue through the tongue and through the bottom of the floorboard into the joist. Do it such that when screwed fully home, it doesn't interfere with the tongue into groove fit of the next board, i.e. the screw head must be countersunk into the base of the tongue.
These screws work really well as the bottom coarse thread screws pulls into the joist while the top finer thread pulls the board really nicely down such that it gets rid of any cupping in the board.
It's a good idea to have acclimatised all the boards by storing them for at least a week in the room where they're going to be fitted, so that their humidity level is somewhere close to that of the room. If they're really wet, or really dry, they may split when fitted into a room which is probably quite warm and with reasonable levels of humidity.

 

Timber is so variable these days, I would use 2 screws on each board at each joist. Tends to help prevent cupping.

No matter how tight you clamp up new boards, after only a week acclimatizing, I expect they will still shrink a bit after a good while in a centrally heated house, so 2 screws helps to hold each board firmly down.