Magi

I am trying to find the correct size for my tyre so far i have had three different numbers suggested in the drop menu's of many companies i need to buy a tyre for a renault Megane Dynamique 1.6v 2003 it is a 2002 model but was registered in 2003 this is causing havoc as i cant get any of the numbers of the parts without pysically getting inside the car I am due for an MOT the first one failed so far i have 1856015XL for back tyre but this seems to differ and i assumed all the four tyres were the same size the last two numbers at the end do not show up ie 15 for my make and model of car on many drop menu's i cant afford to get this wrong as so far i have a bill of £500 and a budget of £400 which consists of £112 for the Cat Converter etc Lamda sensor pipe coil etc etc and i cant afford it! The Renault dealers will not give me any numbers of parts there answer yesterday is no sorry we cant do that as you could then go and buy that part..... DOE! It's a long shot i know but can anyone help or is this a Conspiricy or some kind of Brotherhood between Mechanics PS i live in Highbridge Somerset Thankyou All