Try ferreting around the site below (Worksmart) which is sponsored by the TUC so it should be fairly OK and up to date. The link I have set up relates to temporary / permanent employees, but there's plenty of other material in the same area of the website of potential interest to you.
http://worksmart.org.uk/rights/viewquestion.ph p?eny=807 The terminolgy you should be using when searching is 'temporary' employment, not 'casual'. Not sure about new legislation, but there has been an issue for many years over temporary employees gaining the potential to permanent status after 2 years service - I saw it first in 1989 when working as an HR Manager. The issue then was that a temporary employee could claim the same rights of a permanent employee, for example membership of the company pension scheme, holiday pay, sick pay and unfair dismissal rights. However some temporary employees may get some of these benefits anyway - it depends on the companiyes policy. However its not good practice legally or in terms of HR policy to retain such staff for long periods under the basis of regular, part-time employment. If you are really using a person on the basis of a few hours here and there, I don't believe they have any such employment rights because you commission them on a unique basis on each occasion. Have a look in the Worksmart site around the indicators that point to 'employee' or 'worker' status - such things as whether you pay their Class 1 NI. That should help you.
Regarding line management practices, surely its good practice to follow a similar set of processes as for permanent members of staff regarding performance review, likewise sickness (from which I assume such employees are eligible for your company sickness payment, not just SSP)?