Sorry guys, but the ISA system is not going to replace CRB Disclosures. It is going to be as well as CRB Diclosures!
The ISA will be a one-off registration, which future employers/volunteer co-ordinators can inspect to make sure you haven't been banned from working with children. It does not comment on whether you are suitable for the specific job you are planning to undertake. That will remain the function of the CRB check, for the foreseeable future. (Although I have heard that they are making plans to have the CRB checks constantly updated, so you don't have to keep renewing them for each organisation.)
The reason you have to do a different one for each organisation is that the information provided by CRB can be different for different roles. The classic example is that a role which might involve driving children around will attract a check on driving offences, whereas those wouldn't be disclosed on a CRB check for a different job.
More seriously, it was recently explained to me that Disclosures for certain jobs might include information about other individuals living at the same address as the person applying for the Disclosure. Because, for example, you might invite children to your own home (for a social event or something). Whereas a teacher, for example, is highly unlikely to do that.
Since I know someone living locally who is unable to have children in her home because of something her husband has done, I now begin to see the value of having specific checks for each job. As someone who co-ordinates volunteers for church-based children's work, I would no longer accept a Disclosure obtained for a different organisation, because I cannot assume that it would have covered all the right information for the type of role I am recruiting for.
And all the forms I've seen have been only four pages long, of which you only have to complete two and a half. So I don't know what other form you've been given - maybe it's specific to Beavers?