Qualifying Cross Country Validity
 

Is there any time limit on the validity of the qualifying cross country for an EASA PPL?

I have looked in CAP 804 and I can see nothing that indicates that the QXC has any limitation on its validity once completed, but I want to be sure.

It's not a 'qualifying' cross-country and there is no validity - once you've done it, you've done it.

SRG 2105 "qualifying cross country"

6 months I'm guessing as its a course requirement and all course requirements must be met within the 6 months immediately preceding the LST?

That cannot be correct.
Passes in all Theorectical Knowledge exams is a course
requirement, and once all are passed they are valid
for two years.

Can you tell me where CAP 804 says this? Or is it in a different document?

This is a meaningless document. Its origins lie in the old National AOPA syllabus which continued to be used long after the demise of the UK PPL. The Qualifying Cross Country was a flight conducted as part of a UK PPL prior to 1999. This flight could not be undertaken until after the Navigation Skill Test had been successfully completed. In many cases it was completed after everything else including the GFT, and was thus referred to as a "Qualifier" in those days it had a validity of 9 months. Because it did not form part of the "training" it required signatures to prove it had been completed. That ceased with the introduction of JAR-FCL. It is an experience requirement that has no validity period!
Once you have it you have it! There is no EASA requirement for any Certificate other than the training records but the CAA seems bogged down in things it clearly does not understand.