I found Santander to be the worst of my accounts when I came to Thailand, they froze my card very quickly. Luckily I most of my money was in other accounts.
As Lynnette stated (and other people have said it to me previously) Nationwide seem to be the most lenient, so maybe try them.
I have extended my stay 3 times now and on average it has taken me 45 minutes.
You go to immigration and get a list of requirements.
You need to complete a very simple A4 form with questions like; name, passport number, date of arrival in Thailand etc.
You supply the documents on the immigration list and pay 1,900 Baht, wait a few minutes and you are done for another year.
I personally don't think it's worth using an agent for this simple task and immigration are helpful.
As for circumventing the law by using an agent to deposit the funds, this is a mistake IMO as it is illegal and people do get their Visas cancelled for this reason. It's up to the individual what they want to do.