In article ,
says...

Is there any way to do this properly?

Dig it out and repair a nice square sided section
--
Thanks, will a shovel cut tarmac?, if not I gues I can use my angle
grinder.

Use a bolster and lump hammer to cut a clean edge, a spade should do the
rest.

But where do I get the bitumen sealant from - I've tried and never find it.

Roofing suppliers?