So at long last the man cave resurrection is underway. It didn't take long to find out why the front was sinking and causing structural cracking - there are zero foundations at the front ! The sides and rear are OK (test holes) but the front is just built on clay... Muppet builders of the highest order. Still, a hefty dose of concrete has sorted that so we can rebuild the front, raise the floor up three brick courses to give a nice, level drive and move the lintel up to suit.
I get to keep the same size garage with a little bit less headroom - happy.