Tyres - Matching tread depth??

[Siy]

Wife had a small prang in a 13 plate Merc A180... It has just been repaired, and the nsf tyre had been damaged in the prang, so was replaced (with the same tyre that is on the rest of the car).

However, that now means one front tyre is brand new, and the otehr os well worn... probably a couple of mm left on it.

Question is should I replace the worn tyre, so both fronts are brand new, therefore have same depth of tread etc?

thanks

[Siy]

Will obviously replace once it gets to 3mm, so just wondering really if its best to do that now?
Same tyres all round - rears are good... fronts were getting low, and with one just replaced, in my eyes, means they dont match.

[Siy]

great - thanks guys... when i said it only has a couple of mm left - I meant left until not legal.

I will get it changed in next week or so
Thanks