NOT STARTING

I have a 2007 combo 1.3cdti that i use for my dog walking business. Every now and then,it doesnt start. Dash lights
come on but does not tick over. I have to wait a couple of minutes,sometimes 10-20 mins then it will start. I got it towed to the garage 3 weeks ago when it refused to start after 20 mins of waiting. They replaced the alternator - £300. A few days later it did the same thing. i called the garage to rescue me,when they got there,it started! They said it started every time they turned it on and could not find a fault. I collected it and the next day it broke down again and would not start in the middle of town! We jump started it and drove it to nearest garage. They kept it all weekend but it started every time they went out and tried it! They've told me to park it on a hill (not all my customers live on hills!) and also to leave it on (with 7-8 dogs in the back!). They cannot look for a fault until it breaks down (which it did today with 8 dogs in the back!) After 15 mins or so it started. So frustrating! Can anybody help? Did I need to spend £300 on an alternator in the first place? Many thanks in advance.