I didn't pick up on the householder loaning the ladder - that does put a different slant on it - IMO.
Perhaps there is blame on the householder and the business owner proportionally - and yes the labourer although pressured by the owner (tacitly)and the householder (directly) could have exercised the right not to climb. But overall, in my view the labourer should have been protected primarily by the business owner. But is there any money available to be claimed there?
But I understand how it could have happened - fussy custy getting the job on the cheap by not getting workers equipped with the right gear - obliging workmen acceding to her demands - perhaps the ladder just needed footing?
Judge Malky Solomon now rules:

Claim £100,000 (say)
Knock that back to £80K for being "adventurous":-
Householder (through their insurance) £20K
Business owner £40K
Labourer £20K
So labourer bears 25% Householder 25% Business owner 50% responsibility.
Labourer gets 60K
Insurers cough up, precedent set, lesson learned.
No insurers? Ooh err Missus!