Thank you all for your lovely comments :)
1. Clean off old dirt, wax, polish etc with methylated spirits and medium grade steel wool. Use an old towel to wipe away the dirt. Keep doing this till all/ most of the dirt comes off.
2. Sand. Firstly with a heavy grade and getting lighter as you go. All depends on the staining and wood. Mine is oak so I could use a heavy grade but if you have a piece without much problem areas you could either just use a lighter sandpaper or a fine wire wool.
3. French polish. I used Rustins clear French polish. You will also need cotton wool, muslin and a jar! You would see how to apply on youtube.
4. I varnished the top in a clear matt varnish as I don't want to have to be too protective over using the table (as with French polish). I can't find the name of that one but any good hardware will advise you.
Hope that helps and PM if you need any more advise. :-)