Issue with ka.. Need advice!

[Hayley1983]

I'm new on here but desperate for some advice. I bought a ka about 7 weeks ago and have had an issue with it 3 times now (third time today!!). Basically I'm driving along then suddenly the brake goes very hard and I can't accelerate properly (can only drive slowly in 1st or 2nd gear), it's as if the car is struggling once the brake goes hard. I pulled over and you could smell something horrible from the drivers side wheel. The guy I bought it from is a mechanic and has twice repaired this for me but it has happened again! Last time he said he changed the disc and the caliper, however the driver from green flag today said the drivers side disc is completely worn. 

Does anyone know what could be causing this? What's strange is that once the car cools down the brake is fine again! He has offered to fix it again but  thinking of just getting my local garage to take a look as I don't know whether I trust him as this is the third time!