Currently one of the big issues consuming a lot of time at many students' unions across the country are the recent changes to Charity Law, including the requirement to have clearcut definitions of trustees. There are a lot of myths flying around about what the act does and doesn't require, and frankly some attempts to sneakily slip through other changes that are not required by charity law, but this isn't really the place to discuss those.
One common proposal is to have a trustee board, made up of a mixture of some Union officers, some ordinary elected students and external experts. My former students' union, Kent Union, is currently holding a referendum on such a model and, although I've not been following the internals of Kent Union for the last few years, I have to say this proposal is a strong improvement on the current set-up.
The last major constitutional rewrite (carried out in 2001-2002 by a working party including myself) frankly really only rearranged some officer positions, modified the decision making bodies to be practicable and semi-admitted that de facto the university and student body isn't really collegiate anymore (see my past comments on this in From Vision to Reality), but didn't really grapple with some of the more basics such as the often difficult relationship between democracy and charity law. This was because none of us on the working group really knew very much about the details of it, even though there had been an incident over charity law at the start of the year.
The said incident would undoubtedly have been handled better under this proposed new set-up. The Union affiliated to the National Abortion Campaign in late 2000, with not a little controversy and one individual was determined to end the affiliation by any means possible. Flash forward to the start of the new year and the incoming President Alix Wolverson received a letter from said individual citing dodgily obtained legal advice and comments from someone at a Scottish students' association that said the affiliation was probably beyond the Union's legal scope ("Ultra vires"). I know what some of you are thinking - Scotland has a different legal system so cases are not always automatically precedents for England & Wales. And "is probably" is not clear cut advice.
However the decision on this was taken by what was then the "Finance and General Purposes Committee" made up of just the six sabbatical officers. They didn't all have a training in law (from recollection at least five if not all six weren't even law students) and some didn't even know what a Constitution was (including Alix Wolverson, the President!). As a result a decision to disaffiliate got taken which proved messy because a) those dissenting didn't agree with the advice or its basis; b) the committee didn't actually have the powers to make this decision under the constitution; c) the decision was announced to the wider student body (and even the executive) in a very bad way, with the result that very few even understood the reasoning behind the decision; and d) there were subsequently attempts to reinstate the motion, including one that said that the Union should go all the way through the courts over this affiliation. (The fee was a mere £65 - all perspective was lost!)
Had there been a properly defined trustee board with expert members on it, and with continuity from year to year, then both the original decision to affiliate and the subsequent decision to disaffiliate would have been subject to expert advice and opinion from the outset, the mechanics would have been a lot easier to explain to the Union at large and it's probable that one of the two decisions would not have been made. Whilst diehards will always be diehards, the Union as a whole could have got on with more important matters without having to deal with a lot of bad feelings because of the way the matter was (mis)handled).