I bought a bike with a set of very squealy Deore M615 brakes, with very little bite. I assume the pads are contaminated.
The pads look pretty new, I’ve removed and cleaned, but no joy.
I removed and blow torched, but no joy, then removed and sanded, still no joy. Discs have been cleared with Muck Off disc brake cleaner, still nothing.

Anything else I can try?
If not and I replace the pads, will I need to replace the discs as well?

No, just clean the disks. But if new pads only work for a bit, then you have a leak and need new calipers.

Douse the caliper and disc with washing up liquid ride dragging the brakes for a minute or two. Rinse and repeat as necessary.

As others have said, replace the pads but just use something cheap-ish and clean up the discs with brake cleaner or similar. If they start squealing again after a short period (couple of weeks, say) then you’ve probably got leaking calipers, which aren’t unknown with Shimano brakes and you’ll need to sort this out before you burn any more money on pads.

Are they squealing without applying the brake? in which case, calliper alignment issues.

Squealing after a long bit of braking? smear a little copperslip on back of pad (avoiding contact with the pad). Though when I get that I find it only does it when not applying the brake.

But if it’s struggling to bite as well it sounds like a leak. Especially if after cleaning it’s biting fine but gets worse and starts squealing.