The wording doesn't even include different aerodromes anymore!
Yes it does:
Part-FCL Appendix 3
Training courses for the issue of a CPL and an ATPL
E. CPL modular course — Aeroplanes
EXPERIENCE
12. The applicant for a CPL(A) shall have completed at least 200 hours flight time, including at least:
(a) 100 hours as PIC, of which 20 hours of cross-country flight as PIC, which shall include a VFR cross-country flight of at least 540 km (300 NM), in the course of which full stop landings at two aerodromes different from the aerodrome of departure shall be made;
I cannot find written evidence that the aerodrome triangle can be less than 300nm
There isn't any. It doesn't have to be. However I think it is a good idea as aerodromes (usually) log landings so, provided they are the requisite distance apart, there is (if necessary) an independent means of verifying the flight met the distance requirements.
Therefore, I was thinking of leaving Tatenhill mid-afternoon, flying to Jersey and staying overnight, then flying in to Guernsey the next day before heading back home... is this allowed?
The requirement is for "a VFR cross-country flight" ie one flight. There is no definition of what constitutes one flight, as opposed to two (or more) different flights.
Time between landing and taking off would probably be a consideration. I don't think anyone would argue that a Take Off one week after Landing is a continuation of the same flight. An overnight break: Who knows? But I would suggest these now become separate flights - so not allowed for the 300nm experience requirement for CPL issue.