Got asked about this earlier and I have no idea so was hoping for some direction.

The gunk is not actually in the coolant itself, just around the rim/neck and inside the lid. It's quite thick, gritty and gunky, and appears to have metallic flakes in it?

That will be rust. Just shows that at some time your car has been run with just water or a very high concentrate of water in the coolant system.

You can buy coolant bottles brand new if you want to change it.

Does the coolant itself look clean enough? Not dirty or cloudy?

yea probably rust due to not enough coolant , could also be radweld type products in there too

Got told it was clean and is now around the 'minimum' mark. Guessing it's had too much water in the ratio. So can it just be topped up with coolant or will it have to be flushed?

If you have never done so I would flush the system just so you know what is in it. Especially in this cold weather as you don't want to risk the water freeing in the block.

Was last flushed in April I believ

As Steveo says, it sounds as if there may be some 'Radweld' type of stuff in there.

Regards