Building company next door won't pay agreed share of retaining wall

Hi all, would love some advise on this very stressful issue which has been going on since November 2013!
Background: we built investment property in Toowoomba, retaining walls were required and priced in our contract, then days prior to our builder erecting a 1m retaining wall, the builder next door cut their block approx 1m down on our boundary, resulting in an issue with installing the wall (we are on the higher side). Essentially now requiring a 2m wall. Both builders agreed it was in both parties interests to build 1 x 2m wall and costs would be split. Surprise, surprise, we still haven’t been paid. But have learnt a lot over the past 6 months!!! (now aware of the agreement we should have completed and signed prior to all of this)
Long story short and months of phone calls & emails, the company (Construction Build) have agreed to pay us $4300 of the $12000 total (fence & wall). Unsure where that figure came from but we wanted it over so happy to accept.
2 months after agreeing to pay $4300 (which we have in writing), they have paid us $2000 and said would pay the balance the following week. Which they haven’t – 1 month on.
Any advice on how to get this $2300 from the company please? It’s stressing my wife out immensely and we really just want to put it all to bed but it’s all cash. They won’t call back or answer when they see our number calling. Should we go to the Small Claims (QCAT)?….
Appreciate any help anyone can offer.
thanks,
Scott