Happy to report a successful border crossing. Managed to get a return bus from CM for 365 baht, I would advise booking in advance as I turned up at the bus station around 12.30 yesterday and the next available seat wasnt until 5pm which didn't get into Mae Sai until 10pm after the border closes at 9pm. I had the same problem trying to get back to Chiang Mai last time, turning up at the Mae Sai bus station at 9am and not getting back to CM until 10pm so make sure to book in advance! (I also have experience booking and paying for a Green bus ticket online and then being called on the day i was due to travel to be told the bus was fully booked and there was no space for me. So better to buy a physical ticket from the bus station or a travel agent.) The bus station in Mae Sai is quite far from the border so you'll spend 30 bhat on the song taew each way in and out. And even spending 350 on a hotel for the night(Mae Sai complex you can find on Agora and is really close to the border, which opens at 6.30am fyi) it's still a much cheaper way of doing things. I recommend changing some baht into dollars in advance as it's either 500 baht or $10(which is around 340 bhat) for the Myanmar day pass so you can save a little here but I totally forgot this time. I was politely reminded that it's a maximum of 2 land entries per year as this was my second.

I'm on the way there now via bus, much cheaper

Well i will give it a try and report back. Thanks for the comments guys.