System error

I booked my flight online i wanted to fly into mumbia and out of delhi not knowing how to do this i called airindia they explained that I had to click on the multi city option so i did this and paid by credit card when i got my itinerary my flight was in and out of Mumbai not what i wanted so i contacted airindia by phone they told me it was a system error and i jad to contact airindia.in they will change it so i emailed them and got a reply what do you want us todo well if its not obvious you shouldn't be in your job so when i arrived at Heathrow i went too ai counter to explain my situation they told me its 25 quid to change here or nothing in mumbia so hey ho mumbia i got given airindia.in email again so after another 10 emails i got no reply then i tried to call then i got know where apart from pay money like they havent got enough i travelled this time 3 times each time my flight was delayed ive read online you can take them to small claims court exactly what im doing when i get home this airline is useless the worst ill never fly with them again horrible experience