The method of calculation, which I've described, (or something similar to it) is used by 'responsible' employers (such as health authorities) who employ staff on 'nil hours' contracts. It must be noted, however, that some (possibly most) people who are employed on 'nil hours' contracts are not legally 'employees' of the companies with whom they have such contracts. In such cases, people with 'nil hours' contracts do not have the same rights which 'normal' employees have. In particular, it's possible that a person with a 'nil hours' contract may have no entitlement to any paid holidays. (A test case was brought before the courts in 1998. The Court of Appeal ruled that the people involved in the case, who were on 'nil hours' contracts were employees of the company with whom they had such contracts and therefore entitled to the same treatment as other employees. However, in 1999, the matter was referred to the House of Lords where it was decided that the people on 'nil hours' contracts were NOT employees and therefore had no rights to the statutory entitlements of employees. Reference:
http://www.swarb.co.uk/lawb/empNilHours.shtml) .
To summarise: If you're on a 'nil hours' contract, you may have no right any holiday entitlement at all. If, however, your contract is with a 'responsible' employer, it's likely that your holiday entitlement will be calculated by working out how much holiday entitlement other staff accrue for each hour worked and awarding you a similar entitlement.
Chris