I think openLDAP should be one of the first products the submitter tries. In my experience it is reliable scalable and free of proprietary cruft. I have used it for years in a commercial network with Samba. OpenLDAP has allowed my company to drastically cut licensing costs, support costs and lengthen hardware lifecycles. As the submitter is UK based I would recommend they contact Sirius. Sirius are the consulting company I use and they are the only UK OGC/Becta accredited FOSS specialist. Sirius have considerable experience in the UK education market and in the submitters position they would be near the top of the list of people to call. Take a look at their client list to see the kind of pedigree they have.

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I have worked closely with Mark Taylor the CEO of Sirius for a long time now. Please consider anything I say about them biased, contact them youself and make up your own mind about them.

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I have been running an openLDAP/GNU Linux production system for the best part of a decade now. I use openLDAP and Samba on Debian. It is my experience that the LDAP implementations from commercial vendors are often proprietary, unreliable and usually do not play nicely with FOSS services that wish to use them. Unless you have a very good reason to use a proprietary implementation might I suggest trialling plain old slapd on one of the free distributions first. You will of course benefit from much lower licensing costs in the long run if this approach works for you.

Documentation is your friend here. When you buy an item of kit create an operating and maintenance manual for it. These can simply be a folder with the hardware manuals and maintenance agreements in them. I also include the emails between management and the IT admins discussing the equipment order rationale as well as the purchase order. Make sure your emails make clear what the kit does not do as well as what it should do in these emails. Ten minutes of printing, hole punching and putting it all into a folder can stop a management witch hunt dead in its tracks for years to come. This is especially true if features have been cut due to management led budget constraints. Adding the relevant CD's/DVD's and build notes can make your life much easier in the future as a bonus.