don't replace it until its rattling inside or MOT tester tells you otherwise

LOL

PMSL whoever told you that was winding you up :lol: the eggy smell is a bi-product (methane) given off when some harmful gases are converted inside the cat. Perfectly healthy.

I'm sure i read somewhere that if you smell an eggy smell in your motor it's to do with the cat going, well, i had this smell at the wkend.

How much would a new cat' be?

I'm considering getting a new cat and a de-cat pipe if it's not too expensive to swap them around for every MOT