Hi,
The problem here is that you have nothing in writing from Smile - it wil all be heresay. This is hugely frustrating for you I realise.
The issue yu have is that if you just pay the £3328.33 and ignore the last payment, in the eyes of Smile, you wil have defaulted. They will then pursue this amount through the courts and could issue a CCJ..... which would be disasterous for your credit rating.
I know it looks like you are backing down, but I would reccommend that you pay them in full. Then I would just never deal with them again.
Don't forget, if a company makes you happy, you will tell 5 people...... if a company does you an injustice....... you will tell 50.
Tim Moss