Hi Dave, i'm assuming that you will be taking in Bangkok while you are there.
So if you are travelling around ie. Chang Mai in the north, Krabi, Phuket, Phi Phi in the south etc. then flying is the reasonable option, cos you are looking at distances 400 to 500 miles south, 300 to 400 north.
Internal flights are cheap, airports well organised.
Car hire from your base is easy, reasonable and a heck of a lot safer than India.
Good luck with your planning.
Eddie

Thanks Eddie.
Yes we will fly in to Bangkok, and having noted comments will investigate Eva and Thai airlines, especially as you can buy internal flights in a package when using them as your main international carrier. Had hoped to use Qatar as we have collected a few air miles with them but prices and transit times do not compete.
I have to say going DIY without having been before is a bit daunting as I haven't a clue were to start looking. Just trying to glean bits here and there. I don't want to spend too much time travelling either. We did a tour of Kerala last year that was organised for us (but agreed to beforehand) and made this mistake. We never really unpacked our suitcase as we moved on after two or three nights nearly everywhere we went. It gets a bit trying after a month !

Dave, if the DIY is making you a bit edgy can understand, first time n' all that. There is a firm called Tropical Collections on the net, have a look at their Thailand Multicentre pages, then ring them up. Really helpful and will alter/tailor any of their holidays to suit your duration in places of your choice. Have used them in the past and cannot fault their organisation of flights, connections, transfers etc.
Eddie

Thanks Eddie, I'll check them out. Sometimes it's better to see a place before going it alone !