SETV definitely the way to go depending on your frequency of visits and length of stay you might later look at getting a Non O visa based on marriage. Presuming of course the in laws are Thai. You download the visa application from the website there are even samples of how to fill in if you google them.Don't forget the PO is £35. You can apply in person but its two visits as they process them for next day pickup.
*********************************************************************************************
*****
**********
-How-to-Apply-for-a-Visa-by-Post.html