Doing the Money Claim Online thing is easy, and not as daunting as you think.
I successfully took someone to court 3 years ago, and once the paperwork was submitted, it was just a case of sticking to the guidelines laid down. So I was ringing them at prescribed times, just to make sure that the case was not getting lost in the amount of claims that they had.
Whatever you do in the first instance of contact with the agency, sod ringing them, and instead e.mail them. That way you have an electronic paper trail, and you can then prove to the courts that you have given them every opportunity to resolve it amicably.
If you are in a union like URTU or any other, then maybe a word with them instead of ACAS might be better, because if you need to obtain the services of a solicitor, then they will more than likely foot the bill and fight it on your behalf.
Ken.