how can i fix my Peugeot 205 DT?

I have a K reg 205 DT that normally starts first time every time. However, if the car has been running and is allowed to cool for a while i.e. 30- 40 min while doing a bit of shopping it won’t start. I can’t even get the engine to turn over not even a click. If i have a short stop ie to get fuel it will start without a problem. When the car won’t start all i need to do is use a set of jump leads and it starts first time. I just don’t have a clue what is causing the problem. Needless to say i travel with jump leads all the time now but would like to go out without having to worry about it starting again. Can you help?