[Lazarus] Strange problem compiling IDE

[C Western]

I am having a strange problem when compiling the IDE (current svn for 
both IDE and FPC). The compilation stops with

Verbose: PPU Source: Expr.pas not found
Verbose: PPU Source: utils.inc not available
Verbose: [lazarus] Error 1
Verbose: [idepkg] Error 2
Debug: make: *** [idepkg] Error 2
Warning: Recompiling Expr, checksum changed for spe {impl}
Expr.pas(78,12) Fatal: Can't find unit Expr used by FormGrid

It was triggered by adding spe from numlib in the fpc rtl to one of the 
units in a component that is in a package I added to the IDE (FormGrid). 
My application using the same set of component components compiles fine. 
If I remove the dependence on spe the IDE compiles fine. With spe in, 
both the Expr.pas and FormGrid.pas are compiled fine as part of the 
build process when the packages they are associated with are compiled.

Running the IDE compilation with -vt to try and diagnose the problem is 
not that helpful because of the volume of output, but I can see that 
spu.ppu is only looked for in a few places, not the standard place for 
all the fpc packages (which certainly do feature in many other searches.)

Any hints as to how to diagnose this? Why is it trying to re-compile 
stuff that has already been compiled earlier in the build process for 
the IDE?

Colin