We want to tile our hallway and have removed the skirting board as we want to replace it with tiled skirting. One of the panels of skirting board was inserted below the bottom end of the plasterboard, rather than being attached to it, by the house builders. So when we removed it, we just found a big gap. What would you suggest we could do to have a surface that we can attach skirting board size tiling to? Do you just add a panel of plasterboard (fix it to the wood of the stud wall) and use plaster/filler to smooth the edges? Any help appreciated
See photos...

12.5mm plasterboard (if it's of the size originally used) cut to size, screw in place, skim, job done ;)

You will ideally need a piece of p/b big enough to fill that hole, and a piece of timber about 2"or 3" X 1" X approx 20" long and a handful of drywall screws.

Put the batten in the hole behind the p/b with half of it showing below the existing p/b.

Hold it there (ideally with a couple of speed clamps) and then screw straight through the p/b into the batten to keep it there.

Take off the clamps and put your new piece of p/b in the hole and fix to the new batten and to the wall plate on the floor.

Fill holes, make good plaster, and decorate.

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One day it will all be firewood.

See gap to the right, push between architrave and screw. It's not a large span and it's going to be tiled, no need to go overtop with support ;)

Keep it simple....

From the OPs description it's only a tile skirting, not the whole wall.

How high are the tiled skirtings? Will they cover the joint on the p/b?

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One day it will all be firewood.

The tiled skirting wont cover the transition, it would make the skirting too large and look out of place unfortunately. It it also the first skirting/wall you see when you open the front door, so it needs to look tidy and be steady enough to take some slight scuffs.

I see no reason to amend my original advice, I believe that in that situation the joint at the edges of the two pieces of p/b will benefit from some support. You could alternatively fit a full noggin fitted between the two studs showing; should be pretty much bombproof then.

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One day it will all be firewood.

No need to span such a small gap, It won't move, tile too ridged ;)

Making a mountain out of a molehill.