If like mine it's the starter, jump starting will have no effect. A bump start/tow start will crank over the engine bypassing the starter. Initially I thought it was the earth contacts, but i jacked up the car and hit the starter a few taps with a hammer. It then started confirming (I'm my mind anyway) that the starter was at fault.
Thankfully as you've just bought the car the garage will front the cost. I replaced my starter with a reconditioned unit at £60 and the mechanic charged £108 (Inc vat). Initially I tried changing it myself but there was a bolt that i just couldn't reach. Maybe if i had the hand size of a 5 year old. The mechanic had to strip out a fair bit from the engine bay to get access.
Tom