I bought a fresh dressed crab from Brancaster on Monday will it still be ok to eat now?

Probably not, but if it has been kept at a very low temp. it MIGHT be OK. Use your nose as this is the best test. If it smells even slightly off, then chuck it.

I was thinking this seems such a waste

That may be so, but if it has a bad smell it is not worth risking getting ill by eating it.

Monday to Thursday I'd say not. Well I wouldn't chance it anyway regardless of how it smells.