Wrong size spark plugs

I have a mg zr 1.4 my car was tapping for weeks and i took it to a garage who told me i had to do a oil change and it would go away but no damage was being done the other day then the tapping went away 2 or 3 hours after the tapping went away the plug blew out we have now fitted a 14 x 1.25 helicoil because all the old threads are shot. We bought all new plugs which screw in nicely in our hands the plugs screw into the helicoils out of the car we put the helicoil in snapped the bottom pin of we then tried to screw the plug in but the spark plug is getting cut . The threads on the plug is going flat like a metal ring it feels has tho the spark plug is a little to big for the hole we have rushed t halfords to buy a smaller plug we gave the reg number and she said they was the correct ones for the car but we explained we needed a smaller one and they have told us the zr has the smallest plugs you can get there is no smaller on the thread side i know the helicoil kits come in at 10mm all the way up to 14 and the kit was a 14mm one.So are different size plugs ?What am i doing wrong? I need to find a plug which is a little smaller ?

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