Band castration is supposed to be done within 7 days of birth. The problem with Soay lambs is that their bits are so tiny that they will slip back through the ring, even if you can get them through in the first place.
So, you have a dilemma - do you extend the official time to three weeks (definitely not 8-14 weeks) by which time the testicles might be large enough and have dropped enough for banding, or do you go by the book and get a vet to do it later with a burdizzo or surgical removal.
I am firmly of the opinion that it is a major welfare issue to stick to the 7 day rule with Soay. The bands were designed for strapping great commercial breeds, not for tiny, elfin Soay lambs.
I'm not going to state on an open forum our method, back when we kept Soay, and the choice has to be your own.