Sounds like the wood has moved a bit and the old bitumen is brittle and cracked. I would say that patch repairs will be quicker and easier if you can lift loose blocks without damaging them.
Number them and take a photo before lifting. Clean them up and glue back down in same positions.
I...
55m2 of reclaimed wow that is a lot of chipping. I guess you've done it now. I took up 2 or 3 metres in my hall where some body had relaid after removing a fireplace and installing pipes to a radiator. They hadn't used blocks matching the original and it had all blown.
I never worked out...
