Am very close to rounding out my mech hanger bolts so would like some replacements, please can somebody advise what size/type these would be:

Your best bet is to take a bolt to a local fixings supplier.
They don’t need to be massively strong, as the wheel and qr will hold the hanger in place.
If we could actually see the whole bolt and what frame its for it might help though.

You need to measure what you have tbh. Remember countersunk bolts are measured end to end- usually with bolts you just measure the shank and not the head, CS includes the whole thing.

I’d guess you’re looking at an M4x8 or thereabouts but no substitute for doing it properly. (slightly too long would be better than too short, if you don’t have good thread engagement it’ll tear out)

If it’s a 3mm allen key it’ll probably be M5, M4 is 2.5mm allen IIRC

What’s the bike make and model?

It’s a 3mm Allen key fit so assume it’s an M5 then. Frame is a Stanton, but found a bolt from my old Cotic that fitted one that I lost..

M5 countersunk would be my guess (i for one for my front deraileur blanking plate). Just not sure on the length, but about 5mm or so would be another guess.

Length will be a smidgen less than the combined thickness of frame and hanger, so looks more like 8-10mm to me.

2p