Dreadful refund policy

I had a refund credit note on change of flights from STA. Fair enough, if a little steep...two tickets to Thailand deducted £200-£300 leaving me £850 credit note. If i could re-book with STA i wouldn't lose my charge of £200-300 though so I stayed in with STA trying to sort out flights. What a stressful waste of my time & resources and what a dreadful experience this has been.
I've spent 3 weeks trying to sort out a flight to Central America and being given the run-around but couldn't work out why. No return of my calls, e-mails without pricing then more calls from me trying to get prices. A complete wind-up.
I asked for advice and options trying to avoid visas for Mexico or USA stop-overs. I was finally quoted £800 single, on Friday 19th December by my 'agent' , with no other options or comparisons. I can get return tickets for the same airports for £650 with other more reputable companies. Who are they kidding?
I then receive an e-mail yesterday, on only Tuesday 23rd December to state i either agree or lose another £320 if I don't book by today, Xmas eve 5pm! That would mean a deduction of £500 reducing my credit note from original £1,025 to £525.
Their 'flexible' services aren't flexible. And this accounts for the long delays, avoided my calls, 'forgot' to call me back etc for the last 3-4 weeks. So by doing nothing, they (STA agents) get to take another £320 from me! Clever!
They have given me the advertising spiel of 'we will always be there' and what a 'flexible'company they will be. Baloney!
My mission is to find a match of the flight info STA sent me finally on Friday and get their "lowest flight prices or we'll you £1,000". That's my new aim! And to get a return flight for less than STA can do with my credit note.
Happy Xmas STA team...you've really made my day!!