Do I need a new fuse board *pics*

[OwainDIYer]

Why is the supply to the shower fuse wired into the fuseboard and not taken to a henley block inside the trunking and then to the meter?

Probably because there is no isolator between the suppliers fuse and the fusebox.
Cheaper just to stick it in the fusebox than call out and pay for the DNO to remove the fuse while the work is carried out and refit the fuse after it has been done.

So what's it been wired to inside the fuseboard? The lighting circuit?

 

[flameport]

So what's it been wired to inside the fuseboard? The lighting circuit?

The cable has probably been shoehorned into the incoming main switch terminals, and probably while live as well.

That white fusebox is useless for an electric shower, since the 30A fuse means a 7kW shower or less.

The right hand black fusebox is clearly overloaded, and the internals are likely to be damaged, which may be the cause of the lighting circuit problem.

£300-£400 is probably about right to replace that mess - however given the age of the equipment and someone has already been making dubious modifications, it is very likely there will be other issues which will need to be rectified at additional cost.