Potential bill worry

Hi. I'm very worried how much I may be charged for investigation into intermittent start problem. Towed my car to local garage as car wouldn't start. They rang me to say when first looked at, it was starting, but i agreed they continue to look at it to find the problem. The next day they tell me they ran diagnostics and soldered cluster board and should be okay now. I paid for 4 hours labour and diagnostics. One week later car won't start again and showing same problem. 

Garage tows car back. 2 days later, still not fixed, car intermittently starting, they say.

My worry is how many more hours they will spend on it and expect me to pay more money for the investigation. Already told them I can't afford a lot more money on this car as it's not worth much - plus I don't have the money.!

Where do I stand on this please.? I am at their mercy and getting very anxious and worried.

Thanks for listening.