It should probably be made clear that the register of members will be made available to anyone with a good reason for asking (that's a legal requirement, I don't remember the definition of "good reason").
Good point. I don't think you even need a good reason; if you're a member you're entitled to look, otherwise you may have to pay a fee but you're still entitled to look, unless the company applies to the court to get an exemption. I've put this onto the page, with a ref to the appropriate section of the companies act.
I think the "good reason" bit was added in the Companies Act 2006. I think you have to go to court to get the reason ruled invalid, otherwise you just have to provide it.
I believe holding membership details for 10 years is, indeed, a legal requirement. I don't know of any requirement to keep hold of the actual application form, though.
I viewed the application form as being the membership details. In my opinion, it would certainly be a good idea to keep the bit of paper with the member's signature around, in case we ever need to prove that they applied to be, and are, a member. It also removes the requirement to keep all of their information in the register of members, which as said above can be viewed by all on request.
By "membership details" I meant just the stuff that's on the register. I don't think there's a requirement to even have any more details, so there can't be a requirement to keep them. Keeping the bit of paper wouldn't hurt, certainly, but I would suggest having another copy of the details - keep two databases, one with full details and one with just the legal minimum. Having to go through all the application forms when you need a list of usernames, for example, would get very tedious!