My front rotor has a 4-5 small grooves running all the way round it. I suspect from trapped grit in the pads.

Is it ok to have a few grooves in the rotors, will the bike blow up?

Seriously, should I ignore it, try sanding them down or replace?

The braking performance of the front is pretty poor, I upgraded from Tektros to XT (m765) and didn’t notice any improvement, this may a separate contamination issue though and I’m going to give the pads and rotors a good clean up(levers feel pretty firm so don’t think its a bleed problem).

Rotor is 180mm & I’m 15 stone

You will die.

No, seriously, unless they’re really deep, you’ll be fine. Sounds like they and probably the pads need a good clean with meths or similar.

Depends on the depth of the grooves I would think.
If you are worried stick a new one on – It is your Front brake so quite important.
Alternatively, stick it on the back and buy a 200mm rotor for the front( a 160mm on the back is probably adequate though IMO)