I'm not a lawyer, but here are my two cents:

1. If someone is threatening to take legal action against you, never, ever, EVER deal with their attorneys directly. E.V.E.R. That payment "in good faith" was pure bait-and-switch to get some money out of you.

2. Have you considered applying for debt counselling? It's not a perfect solution, but if your application is successful and you get a good debt counselor, then you will have qualified support from someone who genuinely has your best interests at heart. The fact that you have already been summonsed throws a spanner in the works, but it may still be worth pursuing.

For more info about debt counselling, check out these two links:

Official National Credit Regulator guide
Helpful IOL article