Just had confirmation back from Nam's, bargain, especially compared to the now even higher AOT prices: The price is 1200 bath all tolls included with sedan L. Driver will waiting for you at Meeting point near the Exit 3 (level 2) Please look for her with your name on a card. We will get back to you on the day evening with name of your driver and her picture.   

Thanks @jaredblanc, I'll see how I get on with Nam's but they will be my next port of call. A safe and comfortable drive is a much higher priority having experienced wild driving on Thai highways.

Waiting to hear back from them @Unavowed, the rate is v reasonable on the website. I'll report how I get on if I book with them.

Hey @GeordieG, I use EnjoyTaxiBangkok for my trips

https://www.enjoytaxibangkok.com/transferrates

It's 1700 baht from Bangkok to Pattaya. They can pick you up from Pattaya to go back to Bangkok but you'll need to let them know in advance. 

I've always found them reliable, their 'Big Taxi' transport is comfortable, and their drivers are friendly and don't drive like maniacs. 

In the last year I’ve used Nam’s taxis several times and it cost 1500 between Sukhumvit to Pattaya, and 1200 between Suvarnabhumi and Pattaya. I’m pretty sure that on one occasion the cost was slightly higher because they didn’t have a small car available.