Check track number for NNN/NNN, NN/NN and N/N and change to N

Could someone please help me produce a regex for this so I can be run as part of a combination of scripts rather than having to use the numbering wizard?

Use an action of the type "Format value" for TRACK
Format string: $num(%track%,1)

Thanks

Also, if I want two digits:

Use an action of the type "Format value" for TRACK
Format string: $num(%track%,2)

True to a certain extent:
$num(%track%,1) means that no padding is applied. So if you have a track number "10", then this number remains "10" and is not cut to just 1 digit (be it 1 or 0). The same is true for 100 and following.

I've had problems in the past with playlists with only one digit, learned my lesson and now I prefer to go down this route. I can see from what you are saying that if I had a folder with more than 99 tracks, I am going to have problems?

No, I don't see any problems. The $num() function converts strings to numbers and adds zeros for padding, if set so. Nothing more.