What to do about exposed area of a slab foundation?

[Regis]

Many, many years ago when I was young and (more) clueless,we removed a flat-roof toilet/shower attached my pitched-roof kitchen extension (probably built in the 50s or thereabouts - the flat roofed bit appeared to be a coal store and maybe a pantry or something originally).

We left the existing exposed slab foundation and decked over the top of it. I'm now planning to get the whole extension refurbished properly and have realised that leaving the slab exposed was a very poor idea as it's one big slab under both the external bit and the pitched roof part and I've had the concept of "wicking" explained to me.

It's actually not caused any problems as far as I can tell - there's no damp or anything, but I do want to make it right. What's the best thing to do? I've had the following ideas:

1. Demolish the external bit of foundation - but won't that compromise the integrity of the entire slab?
2. Dig a channel between the external part of the slab and the brick walls and install a linear drain - it would have to be about three bricks deep to get below the slate DPC

Any advice? One builder suggested option 1, the others never mentioned it as a problem at all.

 

[Regis]

I've attached a drawing in the hopes of it making a little more sense. I'm not really sure what is going on below the surface or how far down the slab runs, or indeed, where the DPC goes (just in the bricks?). Hoping someone can suggest what I can do with this bit of foundation.