I used an agent and it cost me around 15,000 baht for their fee and the visa fee.
I had to supply 6 months bank statements, a letter of sponsorship, proof of my employment and a letter / dated photos of how we met and proof of our relationship.
My GF had to send 6 months bank statements and also a letter telling her reasons for the visit and also reasons why she would be returning to Thailand, giving them proof of that
Documents then have to be scanned and signed and personally taken to Bangkok to UK immigration (contracted company) to be scanned onto their system and GFs passport handed in.
Visa was approved and came back within about 9 days.
I have entered on all kinds of visas over the years and was never asked for onward flight. The majority of my visas have been visa exempt either land crossings or flying in through either DMK or BKK.